Add support for a signed field type
Add assembler support for the signed field type
Bugfix: the i field is signed

In the Batbridge architecture the immediate field is signed. This
commit leverages the new signed field function to correctly provide
this behavior.

Also cleans up the parameter naming to match the spec exactly
Patch to reflect new field types
Implement signed immediate values in Batbridge

Use the new signed immediate support to correctly provide signed
immediate values in the tail position of Batbridge opcodes.

Add bounds checking on immediate values so that a user cannot silently
assemble an out of bounds value.
Remove the un-needed signed field special case

Because of how twos complement integers are represented it turns out
that I don't need a special case for assembling a signed field.