vhdl: Stop using trashbox and correctly parse EOF

Although the results of discussion in #90 (trashBox) was technically interesting, I think we are reaching a consensus that it might be better to spend time fixing the parser than to modify it to use a GC mechanism.
This commit implements that fix to the VHDL parser, in line to what was already done in the Verilog parser by @b4n.
No special reason. The main while loop was checking a keyword, so I was induced to also use a keyword without really thinking about its functional purpose.
Please review the updated code.