Removed 16:16 fixed point values from the compiler

The compiler now uses 'double' internally for TK_VALUE and symbol
values, leaving conversion to fixed point to the code generation
stage.

Added a2c_DropToken(), to cleanly move back to the previous token
without backing up the source code position.

Added a2c_Codef() for issuing instructinos with immediate fixed
point arguments, with checked conversion from 'double'.

Added A2_INTERNAL exception for "too deep unlex."

Fixed "safety net" END instruction encoding in a2c_PopCoder().

Various compiler refactoring, fixes and cleanup.

Rounding to 16:16 fixed point has been improved in some cases.

Closes #101.