#4 assembe: make local labels _really_ locals

[Benjamin Gandon]

Local labels (starting with a dot like ".label") are
not reachable from outside the bounding global labels
(with no dot) that are just before and just after.

The idea is to provide a way to invoke local labels
from elsewhere in the code, using the preceding global
label as context identifyer.

Example:

global_label_1
...
instructions
...
.local_label
...
instructions
...
global_label_2
...
jmp global_label_1.local_label
...

In this example, the ".local_label" is called from
outside its context using the preceding
"global_label_1" as context identifyer.