This reverts 425cd7851, which broke the use of casts to define
integer width.
Instead of it, two steps are taken:
* First, literals are monomorphized, leading to predictable result.
* Second, casts are monomorphized, in a top-bottom way. I.e.
consider the expression `int64(round(x))`. If round() was visited
first, the intermediate precision would be 32-bit, which is
clearly undesirable. Therefore, contextual rules should take
priority over non-contextual ones.
Fixes#1252.
d6eb2b023a