Allow pattern matches in `for`, `FlatMap`, closures

[ryzhyk]

We currently don't allow any of the following:

for ((k,v) in map)

(var x, var y) = FlatMap(expr)

|(x,y)| x+y

[mihaibudiu]

Also if ((k,v) = z), which evaluates to true only if the pattern matching succeeds.

[Kixiron]

I would prefer a rust-like if let (k, v) = z {} so it's clear that pattern matching is happening

[ryzhyk]

I am with @Kixiron on this one. if ((k,v) = z) is very unorthodox. I haven't seen this in any other language.

if let (k, v) = z {} on the other hand is just syntactic sugar around match.