auto merge of #14323 : richo/rust/features/strbuf_to_string, r=huonw

Converts `StrBuf` to `String` throughout rustc and the standard library.

Tests all pass locally, but usual caveats about platforms that aren't OSX apply since I don't have a test environment handy.

@alexcritchon mentioned that @pcwalton may have a patch incoming that should block this?

closes #14312
Fix FIXME #5475 in std::ascii

Issue #5475 was closed some time ago, but ascii.rs still contained
a FIXME for it.
auto merge of #14415 : Sawyer47/rust/ascii-fixme, r=huonw

Issue #5475 was closed some time ago, but ascii.rs still contained
a FIXME for it.
std: Remove String::from_owned_str as it's redundant

[breaking-change]
std: Remove String's to_owned
std: Rename strbuf operations to string
Fixup: into_strbuf -> into_string
Fixup to_strbuf -> to_string
fixup: syntax on append
fixup: to_owned -> to_string
Fixup: continued fallout from to_owned -> to_string