More efficient infobool expression evaluator

Expession infobools are evaluated at runtime from one or more single infobools
and a combination of boolean NOT, AND and OR operators. Previously, parsing
produced a vector of operands (leaf nodes) and operators in postfix
(reverse-Polish) form, and evaluated all leaf nodes every time the expression
was evaluated. But this ignores the fact that in many cases, once one operand
of an AND or OR operation has been evaluated, there is no need to evaluate the
other operand because its value can have no effect on the ultimate result. It
is also worth noting that AND and OR operations are associative, meaning they
can be rearranged at runtime to better suit the selected skin.

This patch rewrites the expression parsing and evaluation code. Now the
internal repreentation is in the form of a tree where leaf nodes represent a
single infobool, and branch nodes represent either an AND or an OR operation
on two or more child nodes.

Expressions are rewritten at parse time into a form which favours the
formation of groups of associative nodes. These groups are then reordered at
evaluation time such that nodes whose value renders the evaluation of the
remainder of the group unnecessary tend to be evaluated first (these are
true nodes for OR subexpressions, or false nodes for AND subexpressions).
The end effect is to minimise the number of leaf nodes that need to be
evaluated in order to determine the value of the expression. The runtime
adaptability has the advantage of not being customised for any particular skin.

The modifications to the expression at parse time fall into two groups:
1) Moving logical NOTs so that they are only applied to leaf nodes.
   For example, rewriting ![A+B]|C as !A|!B|C allows reordering such that
   any of the three leaves can be evaluated first.
2) Combining adjacent AND or OR operations such that each path from the root
   to a leaf encounters a strictly alternating pattern of AND and OR
   operations. So [A|B]|[C|D+[[E|F]|G] becomes A|B|C|[D+[E|F|G]].

I measured the effect while the Videos window of the default skin was open
(but idle) on a Raspberry Pi, and this reduced the CPU usage by 2.8% from
41.9% to 39.1%:

          Before          After
          Mean   StdDev   Mean   StdDev  Confidence  Change
IdleCPU%  41.9   0.5      39.1   0.9     100.0%      +7.0%
Faster and simpler portable implementation of MathUtils::round_int().

Much as I like a bit of inline assembler, I have also removed the ARM versions
of MathUtils::truncate_int() and MathUtils::round_int(). The former was just
how any sane compiler should have assembled a cast from double to signed int
anyway. The latter was a much too complicated way to achieve the desired
effect, and was switched out in most ARM builds anyway in favour of the old
portable implementation that used floor().

Verified that MathUtils::test() still passes, and that GCC is now able to
inline MathUtils::round_int(), where it didn't previously.

I tested on a Raspberry Pi with the default theme, displaying the front page
with the RSS ticker enabled. This saturates the CPU, so I'm measuring the
improvement using the debug window's FPS figure. This patch improves this from
~50.8 FPS to ~52.6 FPS.
Improved file buffering in CArchive.

CArchive already did some file buffering, but only on writes. Added the
equivalent code for reads. Also improved the write buffer case so that it
only ever issues sector-aligned writes (the read code does this from the
start). Shuffled various bits of code into the header file to squeeze a bit
more performance out of it.

Profiled the effect on CFileItemList::Archive(), which is one of the slow
parts of the process of reopening a media library, on a non-overclocked
Raspberry Pi. Times are in seconds:

TV shows (253 items)

Before          After
Mean   StdDev   Mean   StdDev  Confidence  Change
0.394  0.005    0.151  0.005   100.0%      +159.8%

Songs (4115 items)

Before          After
Mean   StdDev   Mean   StdDev  Confidence  Change
2.931  0.045    0.690  0.019   100.0%      +324.4%
Add caching of infolabels

The functions CGUIInfoLabel::GetLabel and CGUIInfoLabel::GetItemLabel take
a number of strings returned from CGUIInfoManager::GetImage or
CGUIInfoManager::GetLabel, and combine them with various constant strings
which were determined during CGUIInfoLabel::Parse.

Rather than perform all the string operations on every call, this patch
changes to use a two-pass process: first it queries all the GetImage/GetLabel
strings, and then only if at least one of them has changed does it bother
rebuilding the resultant string - otherwise it re-uses the copy built on a
preceding call.

CGUIInfoLabel::GetLabel/GetItemLabel are also changed to return string
references, rather than forcing an additional string copy.

I have measured the effect while the Videos window of the default skin was
open (but idle) on a Raspberry Pi, and this reduced the CPU usage by 0.8%
from 36.2% to 35.4%:

          Before          After
          Mean   StdDev   Mean   StdDev  Confidence  Change
IdleCPU%  36.2   0.5      35.4   0.5     99.9%       +2.2%
De-duplication of string cache for non-item and item labels