Don't make tags for our dependencies and tests.

Largely, this is just being more specific about where tags get searched
for to remove external dependencies like src/llvm, which reduces the
number of tags *enormously* and significantly increases the usefulness
of the tags file as it is then focusing on 240K lines of Rust code
and 4.5K of C++ rather than just shy of 1M lines of C++ code (mostly
from LLVM) and another 100K lines of Rust tests and a diverse collection
of other languages.

src/rustllvm/RustWrapper.cpp and src/rustllvm/PassWrapper.cpp are
getting tags made, but I'm not sure if that's desirable or not. At
worst, it's not a significant wrong.

A future, desirable step is producing tags for just libstd and libextra
for the use of people using Rust-the-language rather than working on
Rust itself.
auto merge of #10508 : chris-morgan/rust/tags-improvements, r=cmr

Largely, this is just being more specific about where tags get searched
for to remove external dependencies like src/llvm, which reduces the
number of tags *enormously* and significantly increases the usefulness
of the tags file as it is then focusing on 240K lines of Rust code
and 4.5K of C++ rather than just shy of 1M lines of C++ code (mostly
from LLVM) and another 100K lines of Rust tests and a diverse collection
of other languages.

src/rustllvm/RustWrapper.cpp and src/rustllvm/PassWrapper.cpp are
getting tags made, but I'm not sure if that's desirable or not. At
worst, it's not a significant wrong.

A future, desirable step is producing tags for just libstd and libextra
for the use of people using Rust-the-language rather than working on
Rust itself.
Don't mark reachable extern fns as internal

If a function is marked as external, then it's likely desired for use with some
native library, so we're not really accomplishing a whole lot by internalizing
all of these symbols.
auto merge of #10539 : alexcrichton/rust/external-linkage, r=pcwalton

If a function is marked as external, then it's likely desired for use with some
native library, so we're not really accomplishing a whole lot by internalizing
all of these symbols.
pidigits resurected

Changes:
* default value when no args
* license
* removed libc printing
* use extra::bigint instead of handmade gmp binding
auto merge of #10555 : TeXitoi/rust/pidigits-resurected, r=alexcrichton

Changes:
* default value when no args
* license
* removed libc printing
* use extra::bigint instead of handmade gmp binding

The drawback is that it's 2 order of magnitude slower, the good news is that there is a good bench for extra::bigint.
temporarily disable tests on android and tagging issue number #10455
fix #10450
add path environments
android support added to test case