MIPS: Renamed "symbols" to "internalized strings" throughout the code base, in preparation of the introduction of ES6 'symbols' (aka private/unique names).

Port r13781 (59f39a06)

Original commit message:
The SymbolTable became the StringTable. I also made sure to adapt all comments. The only remaining use of the term "symbol" (other than unrelated uses in the parser and such) is now 'NewSymbol' in the API and the 'V8.KeyedLoadGenericSymbol' counter, changing which might break embedders.

The one functional change in this CL is that I removed the former 'empty_string' constant, since it is redundant given the 'empty_symbol' constant that we also had (and both were used inconsistently).

BUG=

Review URL: https://codereview.chromium.org/12374074

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@13807 ce2b1a6d-e550-0410-aec6-3dcde31c8c00