boxing of 'Float' in functions

[gavinking]

Following on from #6614 and #6613, perhaps we could be even more aggressive about avoiding boxing of Number as Float. Currently we avoid boxing:

• within operator expressions or type Float, and
• when specifying a local value of type Float (via = or =>).

In principle, I suppose, we could be even more aggressive about this, leaving Floats unboxed

• when assigned to parameters of type Float, and
• when returned from functions of type Float.

however I need to be much more careful about this because of polymorphism. A function with return type Float can be assigned to the type Object(...), for example.