Pattern support

[utkn]

This PR aims to add pattern support. Patterns are only handled at the extractor side -- all the patterns are converted to either let or let mut bindings.

For example, the statement

let (mut x, y) = (1, 2)

is converted to

let param#0 = `(1, 2);
let mut x = param#0.0;
let y = param#0.1;

[mpenciak]

Looks good!