Support boxed float literals

[JukkaL]

The function i avoids explicitly boxing 12, but the function f emits a box operation:

def i() -> object:
    return 12

def f() -> object:
    return 1.2

Here's the IR:

def i():
    r0 :: object
L0:
    r0 = object 12
    inc_ref r0
    return r0

def f():
    r0 :: object
L0:
    r0 = box(float, 1.2)
    return r0

Make float literals avoid the boxing operation when they are used in a context that requires a boxed value, similar to integers.

[JukkaL]

This should also work for math.pi, etc., that get translated to float literals (see python/mypy#15324).