Add mapper statements

[developedby]

For assignments

x @= f

Desugars to

x = (f x)

And also for map access

x[k] @= f

Desugars to

x = (Map/map x k f)

Where Map/map does get and then set, but without having to duplicate anything.

Then, we can implement in-line operators for maps using Map/map

x[k] += n

Desugars to

x = (Map/map x k @x (+ x n))

Maybe later we can add specialized adder, etc functions for better performance, but for now this is enough.