str: Implement Container for ~str, @str and Mutable for ~str

~str and @str need separate implementations for use in generic
functions, where it will not automatically use the impl on &str.
rm obsolete no-op lints
syntax: print visibilities of foreign items
auto merge of #7932 : blake2-ppc/rust/str-clear, r=huonw

~str and @str need separate implementations for use in generic
functions, where it will not automatically use the impl on &str.

fixes issue #7900
Merge pull request #7936 from thestinger/cleanup

rm obsolete no-op lints
auto merge of #7938 : crabtw/rust/foreign-vis, r=huonw
new snapshot
temporarily disable 3 rustpkg tests

Re-enabling these when the ephemeral failures are fixed is filed as
issue #7956.
auto merge of #7955 : thestinger/rust/snapshot, r=huonw