Improve type inference

[inducer]

For now, type inference falls flat for cases like this:

<> z = 0
z = z + (initially unknown type)

(it just derives the type of zero)

[inducer]

Fixed point iteration might do the job, or else do this properly and implement Hindley-Milner.

[inducer]

Done as of 6995a1ad1aefaa337b289462a67e8f023d2c6501.