Provides reuseable decorator and function to handle magma's implicit
coercion of Python values.

For now, I moved the logic from wiring and mux to a shared place to
ensure consistency.  There's probably other places to do this, but we
can do this on demand as we enounter incosistencies.

One issue is that `int` coercion can't really happen up front like this
since it is determined by the place it's used (i.e. when it's wired to a
Bits we check it can fit in the bit width).  @rsetaluri perhaps we could
use SmartBits for this? (convert it to a smartbits that's automatically
extended to wire to a regular bits, but I think right now our rules
don't allow implicit truncation, but we could define a special SmartBits
subclass that only extends.  My one concern would be whether this would
make the error a bit harder to traceback)

Fixes #828

Provides reuseable decorator and function to handle magma's implicit
coercion of Python values.

For now, I moved the logic from wiring and mux to a shared place to
ensure consistency.  There's probably other places to do this, but we
can do this on demand as we enounter incosistencies.

One issue is that `int` coercion can't really happen up front like this
since it is determined by the place it's used (i.e. when it's wired to a
Bits we check it can fit in the bit width).  @rsetaluri perhaps we could
use SmartBits for this? (convert it to a smartbits that's automatically
extended to wire to a regular bits, but I think right now our rules
don't allow implicit truncation, but we could define a special SmartBits
subclass that only extends.  My one concern would be whether this would
make the error a bit harder to traceback)

Fixes #828