luke's "no implicit coercions" semantics

[dherman]

In Luke's variation of the semantics:

• bits32 is not a subtype of float64
• ints and floats must always be interconverted explicitly via ~~ (float -> int) and + (int -> float)
• variables, parameters, and returns can only have the type bits32 not int32 or uint32
• literals only have the type bits32 or float64 not int32 or uint32

We have the following type signatures:

(_+_)|_ : bits32 x bits32 x bits32 -> int32
(_+_) : float64 x float64 -> float64
(_|_) : bits32 x bits32 -> int32
~~_ : float64 -> int32
~_ : bits32 -> int32
+_ : bits32 -> float64