new experimental feature: control over the native exported name of a function
ref #6285

ccallable(func, [ret_type], arg_type, [name])

is similar to cfunction(), but adds an alias "name" to the global
symbol table exported from sys.so (defaults to the function name).

a method definition can also be prefixed with `@ccallable`

also:
- load sys.so with RTLD_GLOBAL
- shorten jlcall wrapper names to jlcall_f instead of jlcall_julia_f