fix val_eq for casted integer

handle numbers and casts better now u8, u16 and u32 are stored in a respective field in the struct and "untagged" numbers are a plain int64_t.

lowercase cast error messages

fix casting check: 255 is a valid u8!

fix the casting in global_set

allow casts in global vars

drop builtin dummy, add pp and skip

set argc on builtins

change how nativefns are called nativefn (aka built-ins) now gets their arguments on the stack, while script procedures still gets them as variables.

add environments; implement funcall, set/get variables and globals

implement casts

fix compilation when DEBUG is enabled

fix assert in proc_done

yield a message on invalid OPs before aborting

avoid unpleasant infinite loop