Control path is now compiled into ATOM_FUNCs

Now, any list used by AP/RAP is compiled first from a list of symbols
into a list of ATOM_FUNCs, which greatly reduces lookup time: share of
__strcasecmp_sse* calls dropped from 50% of runtime to 13% in
`lookup_symbol()` (and 0.42% in run_secd).

Also, there's no `atom?` in other Scheme implementations, replaced by
R*RS-compliant `symbol?`

TODO: compare strings by hashes, to fasten `lookup_symbol`
Opcode table removed from global environment