better type checking in the VM
better type checking in the VM

Fixes http://code.google.com/p/chibi-scheme/issues/detail?id=5
Some non-opcode primitive functions may still need type checking.

holmescn/chibi-scheme,norton/chibi-scheme,holmescn/chibi-scheme,norton/chibi-scheme,norton/chibi-scheme,norton/chibi-scheme,holmescn/chibi-scheme,norton/chibi-scheme,norton/chibi-scheme,holmescn/chibi-scheme,holmescn/chibi-scheme