Support \u escape sequences in double-quoted strings

Summary: \u escape sequences, when followed by a sequence of hexits
indicate a specific unicode character which will be represented in
the string as UTF-8.

For example "\u{00DF}" is equivalent to "\xC3\x9F"

Reviewed By: @JoelMarcey

Differential Revision: D1729476

Signature: t1:1729476:1418420831:e28cc4c27313a7b1c71828329db6daaa42244e32
Factor out tvRefcountedDecRef calls from ext_collections.cpp

Reviewed By: @alexmalyshev

Differential Revision: D1739064

Signature: t1:1739064:1418670053:01f4697718ed0e1128d6def0eec8fbd4e3724e87