cleanup
call/cc must save pic->ip
update xfile
xstdout now handles its own buffer. flush prompts before printing result expression.
update xfile
update xfile again
update xfile
rename pic_symbol_value to pic_sym_value
add type aliases for some important types
s/struct pic_string/pic_str/g
add some string utility functions
move sturct definitions for basic types to type-specific headers respectively
add type alias for pic_blob
change gc heap API and hide sturctures.
add 'pic_' prefix to heap manipulte functions
fix #58