compile() becomes replacement for builtin compile()
compileFile() generates a .pyc from a .py
both are exported in __init__

compiler.parse() gets optional second argument to specify compilation
mode, e.g. single, eval, exec

Add AbstractCompileMode as parent class and Module, Expression, and
Interactive as concrete subclasses.  Each corresponds to a compilation
mode.

THe AbstractCompileMode instances in turn delegate to CodeGeneration
subclasses specialized for their particular functions --
ModuleCodeGenerator, ExpressionCodeGeneration,
InteractiveCodeGenerator.