p2: restructure, add cmdline -I, potion_loader_add.

move frontends to front/
move syntax to syn/
change more options: removed -c, -s, -X, added -J
rename LOADER_PATH to @INC
rewrite readline evaled code to perl5 syntax
p2: jit simplification, only one target

no cross-compilation, hold only one target at a time in memory (objects were purged before).

Rename type NilKind to Undef
Minor Makefile cleanup
p2: rename nil to undef
p2: add defined method
p2: comment only (reserve --compile-exec and --compile-c)
p2: api test. test defined undef.

defined any fails stills (method not defined yet)
sync changes from git://github.com/perl11/potion.git