Support named tuples

[nystrom]

Fix #95

Introduces the following patterns:

(; x)       -- match a named tuple with field x
(; x::Int)  -- match a named tuple with field x of type Int
(; x=p)     -- match a named tuple with field x that matches p

The first two patterns bind the field to a variable x. The (; x=p) pattern does not bind x.

[gafter]

👀

[gafter]

A couple of minor comments. Sorry I took so long to review.

[gafter]

👀