Add do shorthand for loops (and maybe more?)

[ecton]

To avoid blocks in some cases, we've allowed some shorthands. The primary example is if a then b as an alternative to if a { b }

I'm rethinking the choice of then in that situation, and thinking that maybe we should use the do keyword in if as well as in loops and other locations where it makes sense.

if a do something();

while condition() do something();

for x in expr do something(x);

try something() catch => handle_error(it);
try something() catch do handle_error(it);

The try example at the end shows how the current "fat arrow" syntax already makes its syntax concise. This proposal may be to make do valid wherever the fat arrow is. However, the fat arrow is not as flexible... if a => b else c looks weird to me, but if a do b else c doesn't.