Unify externs, intrinsics, and generators

[Verdagon]

An extern:
extern func myFunc(a int, b int) int;

A generator:
extern("myGenerator") func myFunc(a int, b int) int;

We could make those the same kind of concept.

In the end, we want to be able to change this:

func +(left int, right int) int { return __vbi_addI32(left, right); }
extern func __vbi_addI32(left int, right int) int;

to this:

extern("__vbi_addI32") func +(left int, right int) int;

The only complication will be determining whether it's defined by frontend (IOW a generator, eg vale_static_sized_array_free) or backend (IOW an extern, eg vale_lock_weak).

Perhaps everything beginning with __vfi ("vale frontend intrinsic") could be from the frontend, __vbi by the backend, and __ve (vale extern) by vale-supplied C code.