Open coding of primitives: flesh out the type and range checking,

Open coding of primitives: flesh out the type and range checking,
which was previously a little spotty (e.g. general-car-cdr had no type
checking).  Improve handling of `string-ref' and `string-set!' so that
they inline code in the computed index case.  Flush inline coding of
`char->ascii', which was incorrect anyway since it didn't check to see
if the character was in the ASCII range.