v0.2 Changes

Some work was done on a bootstrapped version of the compiler, but it's not complete.

What doesn't work: register spillover, advanced l-values.

New features:

Fixes bug #249: Don't completely delete foo=bar() if foo is not used
Fix block scope by attaching a symbol table to the BlockNode. Closes #169.
Fixes #263: when compiling IfOp if the operand is a constant, hard-code the jmp
Detect printing of void values as an error. Closes #250
Changed literals to a new TokenType: LITERAL. This fixes #211
If IAE is thrown, catch it and log as a regular D2 exception in the static checker. Closes #217
Make sure the user is not trying to add null to a string, in the type checker and the arithmetic optimizer. Closes #221.
Rewrote null string comparison generator. Closes #218. ONCE AND FOR ALL.
Double-check for compatible array types, and other places where a symbol might have already been defined. Closes #214.
Fixes issue #260: properly compare index to string length.

Fix bug in ConstantPropagationOptimizer: if not incrementing a constant, clear the assignments, just like a transfer. Closes #259.
Fix constant propagation optimizer to properly clear the cache when an assignment is seen. Closes #245.
Implement TempPropagationOptimizer, which simplifies simple binary operations. Closes #178
Include inc and dec as an input to the AdjacentArithmeticOptimizer as i=i+1 and i=i-1. Closes #229
Implement optimization for increment then decrement to two nops. Closes #228
Expanded AdjacentArithmeticOptimizer to bit and, or, xor. Closes #233.
Rewrote parts of constant propagation optimizer - and wound up adding "limited propagation of global constants"! Closes #93

Renamed EQ to ASSIGN.
Moved x64-specific code generation code to the x64 directory.
Added "--target" command line flag. Right now only x64 works. Closes #232
Fix string code generator to use a temp when a string is concatenated with itself. Closes #246.