auto merge of #9035 : alexcrichton/rust/fewer-clownshoes, r=huonw

auto merge of #9035 : alexcrichton/rust/fewer-clownshoes, r=huonw

This removes another large chunk of this odd 'clownshoes' identifier showing up
in symbol names. These all originated from external crates because the encoded
items were encoded independently of the paths calculated in ast_map. The
encoding of these paths now uses the helper function in ast_map to calculate the
"pretty name" for an impl block.

Unfortunately there is still no information about generics in the symbol name,
but it's certainly vastly better than before

    hash::__extensions__::write::_version::v0.8

becomes

    hash::Writer$SipState::write::hversion::v0.8

This also fixes bugs in which lots of methods would show up as `meth_XXX`, they
now only show up as `meth` and throw some extra characters onto the version
string.