Implicit conversion to function types is now a more standard part of
the type conversion infrastructure, rather than special cases of things
like FunctionSymbolExpr immediately returning a pointer type, etc.
Improved AddressOfExpr::TypeCheck() to actually issue errors in cases
where it's illegal to take the address of an expression.
Added AddressOfExpr::GetConstant() implementation that handles taking
the address of functions.
Issue #223.
