Improve scanner performance.
Inline lots of the code used by tryFastScanIdentifierOrKeyword.
Speed up scanning of identifiers.

Also, fix issue where we were not scanning a token as a keyword if it was followed by a
unicode literal.
Clean up the identifier scanning code.
Specialied AST node for index signatures.
Remove unnecessary flag.
Update AST order.
Update AST order.
Bring AST more in line with fidelity.
Introduce finally clause to match Fidelity.
Bring ast catch clause in line with Fidelity.
Reorder AST to match fidelity.
Make ast nodes match fidelity.
Add an EqualsValueClause to bring syntax models more in line.
Move AST ndoes around to match Fidelity.
Bring ForInStatement in line with Fidelity.
Move AST nodes around.
Rename ForStatement members.
Bring ForStatement in line with fidelity.
Simplifying parsing of for statements.