feat: elementwise functions (mappers)

[jpivarski]

At the start of this PR:

% fgrep -r 'assert False, "TODO' src | wc -l
149

[jpivarski]

Now it's

% fgrep -r 'assert False, "TODO' src | wc -l
106

(43 TODO's filled).

[jpivarski]

Now it's

% fgrep -r 'assert False, "TODO' src | wc -l
89

(60 TODO's filled).

[jpivarski]

And finally, it's

% fgrep -r 'assert False, "TODO' src | wc -l
62

(87 TODO's filled). Of the remaining 62, 11 are reducers, which should probably come next.