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.
Add 'continue' as a synonym for 'loop'
Add 'continue' keyword to emacs, vim, gedit, kate
auto merge of #9261 : alexcrichton/rust/logging, r=huonw

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.
auto merge of #9504 : brson/rust/continue, r=alexcrichton
auto merge of #9523 : huonw/rust/kud1ing-docs, r=huonw

Collation of @kud1ing's work in #9511, #9512, #9513 and #9518.
auto merge of #9525 : klutzy/rust/obsolete-span-fix, r=alexcrichton
std and rustc: Convert users of c_str to use .with_c_str
std: removed some warnings in tests.
std: implement Container for CString
std: Add benchmarks to c_str
std: Micro-optimize vec.with_c_str for short vectors

This now makes it unsafe to save the pointer returned by .with_c_str
as that pointer now may be pointing at a stack allocated array.

I arbitrarily chose 32 bytes as the length of the stack vector, and
so it might not be the most optimal size.

before:

test c_str::bench::bench_with_c_str_long ... bench: 539 ns/iter (+/- 91)
test c_str::bench::bench_with_c_str_medium ... bench: 97 ns/iter (+/- 2)
test c_str::bench::bench_with_c_str_short ... bench: 70 ns/iter (+/- 5)

after:

test c_str::bench::bench_with_c_str_long ... bench: 542 ns/iter (+/- 13)
test c_str::bench::bench_with_c_str_medium ... bench: 53 ns/iter (+/- 6)
test c_str::bench::bench_with_c_str_short ... bench: 19 ns/iter (+/- 0)