Add float.infinity/0 to the standard library.

Add a constant function to the float module that returns positive infinity.
This is implemented for the C, Java, and C# backends.
The Erlang backend currently does not support it

library/float.m:
	Add the new function.

	Address review comments from Paul about my previous change.

tests/hard_coded/Mmakefile:
tests/hard_coded/test_infinity.{m,exp}:
	Test various operations involving infinity.

NEWS:
	Announce the addition.