Clean up the Parser test suite

Mostly cosmetic fixes, but all tests for Parser.Depth have been removed,
too (as Parse.Depth itself will be soon).
Remove Parser.Depth

This function increased complexity for no justifiable reason.
Additionally, implementing the functionality outside of the
Parser is a trivial undertaking.
_X -> stateX
Create a benchmark for Parser

BenchmarkParser uses "real-life" data - sample JSON taken
from the Twitter API docs.
Rename string unicode states
Declare variables in Parser.Next once
Optimise if statements in the stateValue case