Refactor the logging system for fewer allocations

This lifts various restrictions on the runtime, for example the character limit
when logging a message. Right now the old debug!-style macros still involve
allocating (because they use fmt! syntax), but the new debug2! macros don't
involve allocating at all (unless the formatter for a type requires allocation.
Enclose code in documentation in backticks
base64: backticks for code in documentation
bigint: backticks for code in documentation
bitv: backticks for code in documentation
syntax: Fix wrong span on ObsoleteEmptyImpl
syntax: Fix wrong span on ObsoleteMode
syntax: Fix wrong span on trait fn visibility

Fixes #9348.
Fix ICE caused by my previous patch, that is, if super trait had more
methods tnan subtrait, compiling would fail. I simply forgot to update
variable name. Updated test case , too.
rustdoc: Fix broken struct field search links

Takes the same approach as variants, writes a redirect index page back to the
struct with an anchor to the field in question.

Closes #9524
rustdoc: Generate documentation for foreign items

This slurps up everything inside of an 'extern' block into the enclosing module
in order to document them. The documentation must be on the items themselves,
and they'll show up next to everything else on the module index pages.

Closes #5953
rustdoc: Strip implementations of private traits

Closes #5416
rustdoc: Render stability attributes

Closes #8965
Ensure that skipped items aren't encoded

If an item is skipped due to it being unreachable or for some optimization, then
it shouldn't be encoded into the metadata (because it wasn't present in the
first place).
remove type_use

This is broken, and results in poor performance due to the undefined
behaviour in the LLVM IR. LLVM's `mergefunc` is a *much* better way of
doing this since it merges based on the equality of the bytecode.

For example, consider `std::repr`. It generates different code per
type, but is not included in the type bounds of generics.

The `mergefunc` pass works for most of our code but currently hits an
assert on libstd. It is receiving attention upstream so it will be
ready soon, but I don't think removing this broken code should wait any
longer. I've opened #9536 about enabling it by default.

Closes #8651
Closes #3547
Closes #2537
Closes #6971
Closes #9222
Add 'continue' as a synonym for 'loop'
Add 'continue' keyword to emacs, vim, gedit, kate
auto merge of #9484 : alexcrichton/rust/fix-cfg-inner-item, r=pcwalton

If an item is skipped due to it being unreachable or for some optimization, then
it shouldn't be encoded into the metadata (because it wasn't present in the
first place).
mark globals as internal when not building a library

Closes #9494
Remove the printf{,ln}! macros

These are deprecated in favor of print{,ln}!