Fix signature computation in compile_extension_module.

Add instant_kwargs to signature. Fixes this bug:

from dolfin import *
m2 = compile_extension_module('', cppargs='-O2')
m3 = compile_extension_module('', cppargs='-O3')
assert(m2.__file__ != m3.__file__)
Add test for instant_kwargs compile_extension_module fix.
No need to check -O3 in this ad hoc test.

-O2 is ok as it is Instant's default.
Merge branch 'jan/fix-module-signature' into next