Function statements

[matthew-healy]

Instead of:

let add = |x, y| x + y in
add 3 4

something like:

def add |x, y| x + y

x + y

This should probably just be syntactic sugar initially, to avoid making too many assumptions about how wider statements should work. In particular, function statements can only reference functions defined earlier in the file (or themselves) and must be followed by exactly one expression.