removing 'str'='struct' search synonym

fixes #16384
rustdoc: use raw search in URL

fixes #16385
fixes #16271
auto merge of #16387 : Gankro/rust/doc-search, r=cmr

Fixes str/struct ambiguity (by removing the synonym) and pushes raw searches to the history instead of processed ones. 

# [Live Version Here](http://cg.scs.carleton.ca/~abeinges/doc/std/)