float literals

Floats are commonly used in game programming and other areas.
This patch adds the use of f or F on numeric literals like C#. Boo already uses the term "single" for this primitive type.

a = 0F
b = 1f
c = 1.0f
d as single = 2.0
print(a.GetType()) //System.Single
print(b.GetType())
print(c.GetType())
print(d.GetType())

It uses float.Parse instead of double.Parse to catch values too big for a float:

//works
zdouble = 1000000000000000000000000000000000000000000000000000.0
//error at compile time, too big for float:
zsingle = 1000000000000000000000000000000000000000000000000000.0f