Translate Python functions into Migen statements.

Functions are translated in the following manner:

• identity statements x is y become module.comb.<domain> += x.eq(y) (but not identity expressions!)
• assignment statements x = y become module.sync.<domain> += x.eq(y)
• assignment statements next_state = x become NextState(x)
• if statements become the corresponding objects
• no other statements in the function body are supported
• all Migen expressions are supported

There are four decorators:

• @pigen.statement(module, domain='sys') Translate the function as described above.
• @pigen.purecomb(module, domain='sys') Translate the function as described above, but treat any synchronous statements as an error,
• @pigen.puresync(module, domain='sys') Translate the function as described above, but treat any combinatorial statements as an error.
• @pigen.fsm(fsm, state) Translate the function as described above, but pass the statements to fsm.act(state, *statements) instead of adding them to self. Additionally, translate x = y to NextValue(x, y).