Add the ability to set __FILE__ and __LINE__

Added the ability to set __FILE__ and __LINE__

    __FILE__ = "test.nw"
    __LINE__ = 10            # this is the line number of the current line

This helps when using tool like noweb to create ruby files.

Bugs: You get a warning about "warning: possibly useless use of a literal in void context".
          and the RHS must be an integer for __LINE__ and a String for __FILE__.  The RHS is not evaluated.

The following example would not change the __FILE__ or the __LINE__

    __FILE__ = "A" + "B"      # warning: __FILE__ can only be set to a String, ignored
    __LINE__ = 10 + 1          # warning: __LINE__ can only be set to a Fixnum, ignored

Changing __FILE__ at the top level (in main) does what you want for backtraces to return the correct values.

Changiing __FILE__ in a block does not affect backtrace outside of that block.

The semantics of '__FILE__ = {string}' is confusing as it looks like the assignment will happen at runtime when the assignment happens (or should happen) at compile time.