Shorthand FFI defbind with function renaming

[jeannekamikaze]

This is an idea discussed with @andrewchambers in the chat.

TL;DR is to allow something like:

(ffi/defbind init :symbol "fooInit")

This would be a Janet init function that binds the C fooInit().

Longer story is to consider something like:

# foo.janet
(ffi/context "libfoo.so" :lazy true)
(ffi/defbind fooInit :void [])

Now foo/fooInit is not very idiomatic, so we can define an alias for that function:

# foo.janet
(ffi/context "libfoo.so" :lazy true)
(ffi/defbind fooInit :void [])
(def init fooInit)

Now a user can simply write:

(import foo)
(foo/init)

The explicit alias, however, adds quite a bit of boilerplate when you're binding 100+ functions. The shorthand would allow us to combine defbind and the alias in one line:

# foo.janet
(ffi/context "libfoo.so" :lazy true)
(ffi/defbind init :symbol "fooInit" :void [])