Compiled "is" gives incorrect results sometimes; Compiled "is not" is not supported.

[rubidge]

def f_is(x,y):
    return x is y

f_is(1,1) = True

but

Entrypoint(f_is)(1,1) = False

Compiled "is" seems to work on plain objects (untyped).

Separately, trying to compile "is not" on plain objects produces error "Unknown binary operator IsNot()"

[rubidge]

Presume this is related to #321