add test in Repl where integers are passed in as the Value. they end up being

add test in Repl where integers are passed in as the Value.  they end up being
passed to Const which ascertains how much space is needed.

for proper Type 1 (ast) nmigen language abstraction, a test is needed
(similar to the one in ast.Mux for bool) which converts *only* integers
to Value, preserving existing behaviour.  there was no unit test for
this case