std::rt: require known stack bounds for all tasks.

We just approximate with a 1 or 2 MB stack for native::start.
Remove unnecessary substitution of type params when generating
vtable methods before translating the ref to the method itself.
libextra: Introduce typed arenas.

A typed arena is a type of arena that can only allocate objects of one
type. It is 3x faster than the existing arena and 13x faster than malloc
on Mac.
librustc: Move block contexts into per-function arenas, and
stack-allocate function contexts.
doc: Syntax highlight the cheatsheet
doc: Fix table format in cheatsheet
doc: Title guides consistently
mk: Start testing the cheatsheet
doc: Highlight the testing guide correctly
mk: Fix the naming of the windows installer harder

CFG_VERSION includes a bunch of VCS info in it that is not a valid
filename. CFG_RELEASE is just the version number.
Fixed Gc::clone, implemented Gc::ptr_eq
doc: Title reference manual consistently
doc: Fix URL of libextra docs
Inline reexports in rustdoc

If a reexport comes from a non-public module, then the documentation for the
reexport will be inlined into the module that exports it, but if the reexport is
targeted at a public type (like the prelude), then it is not inlined but rather
hyperlinked.
'borrowed pointer' -> 'reference'
doc: Add libgreen and libnative to the index
doc: Add rustc and syntax to the index
Address some minor points in the pointer guide

cc #11364.
Fixed code snippets
doc: Typos