Fix $goto target position

[tobil4sk]

Closes #184

The old code would use an absolute position for the genrated Jump, but jumps are meant to have relative positions.

This caused an out-of-bounds error when doing further processing:

neko/src/neko/Bytecode.nml

Line 320 in 1eab131

| Jump n -> pop := (pos.[i+n] - pos.[i]); 23

Using the goto helper function here creates a Jump with a relative address instead.

neko/src/neko/Compile.nml

Lines 211 to 213 in 1eab131

	function goto(ctx,p) {
	write ctx Jump(p - pos ctx)
	}