rustdoc: Sanitize links harder
rustdoc: Build the crate config correctly. Fixes #5011
rustdoc: Handle impl method visibility correctly. #5533
rustdoc: Add type bounds to impls
rustdoc: Tweak list style
auto merge of #5549 : brson/rust/rustdoc, r=brson

r?
Fix #4855: handle bot in regionck
Check for null in return_to_mut.  Fixes #4904.
auto merge of #5556 : nikomatsakis/rust/issues-4904-and-4855, r=nikomatsakis

Two small fixes. 

r? @catamorphism
core: Clarify prelude docs. #4556
core: Make sure every module at least has a one-line description
auto merge of #5532 : brson/rust/coredocs, r=brson

r?
Remove unused imports throughout
syntax: Removing uses of HashMap
doc: Remove mentions of oldmap::HashMap
rustc: Remove uses of oldmap::HashMap
rustc: Purge of HashMap, fixing segfaulting cases

Various XXX comments added around to denote copies which when removed cause the
compiler to segfault at some point before stage2. None of these copies should
even be necessary.
test: Remove uses of oldmap::HashMap
Move ast_map::map to LinearMap
std: Remove the oldmap module