Allow specifying types (type annotations)

[sinelaw]

There are a few cases that are unsolvable without type annotations. For example, polymorphic row fields - always generalizing EPropAssign (row field assignment) means inferring overly polymorphic types that may cause errors later when code that assigns a less polymorphic type is encountered.

The only solution is to not generalize EPropAssign, but allow users to manually specify a (possibly polymorphic) type using type annotations.