Allow astype() to infer a type from its first argument

[wixoaGit]

From the DM ref:

1. If astype() is on the right-hand side of an assignment operation, the left-hand side's var type is the implied type, just like with the new() operator.
2. Otherwise, the var type of the first argument is the implied type, just as it is in istype().

The behavior of 2 is now implemented.

Fixes #2521