std: Remove String::from_owned_str as it's redundant

[breaking-change]
serialize: Remove old commented out code
collections: add Show impl test for HashMap
collections: implement Show for Tree{Map,Set}
std: Remove String's to_owned
std: Rename strbuf operations to string

[breaking-change]
doc: Fix link to string

This was missed in 553074506ecd139eb961fb91eb33ad9fd0183acb
auto merge of #14447 : erickt/rust/show-treemap, r=kballard

This is a hodge podge of a couple small cleanup commits. It implements `Show` for `TreeMap` and `TreeSet`, and some removal of commented out code.
auto merge of #14414 : richo/rust/features/nerf_unused_string_fns, r=alexcrichton

This should block on #14323
rustdoc: Refactor structure of html::run

Instead of one giant function, this breaks it up into several smaller functions
which have explicit dependencies among one another.

There are no code changes as a result of this commit.
rustdoc: Freeze the cache ASAP

The cache is going to be used earlier in the HTML generation process, which
means that it needs to get into TLS as soon as possible.
rustdoc: Show all implementors of traits

When inlining documentation across crates, primitive implementors of traits were
not shown. This commit tweaks the infrastructure to treat primitive and
Path-like impls the same way, displaying all implementors everywhere.

cc #14462
rustdoc: Stringify more named lifetimes

cc #14462