replace main with __main

The spim simulator expects a "main:" label and will start at this label.
This commit replaces the user defined "main" function with a "__main:"
label and places a "main:" label at the very top of the program.
translate the symbol table into MIPS code
fix missing tests

<= and >= tests were missing.
translate the quad list into MIPS code
change output files

Debugging output is redirected to yyout.
If a second file path is provided at the command line, the MIPS code is
written into that file.
update instructions
Merge branch 'mips' into dev

Partial generation of MIPS code.