ENH: Allow type annotations before csp.start/alarms/state blocks

[aadya940]

Fixes #664

The parser now skips whenever we have an ast.AnnAssign with node.value = None while locating the special blocks. Hence, the following cases will be allowed:

x: int

with csp.start():
    ...

with csp.alarms():
    a = csp.alarm(int)

x: int

with csp.start():
    ...

This still raises:

x: int = 5