Add do-notation

[janmasrovira]

• Depends on #2354.

We would support three kinds of statements (bind, let, expression), and we could borrow the syntax from haskell.

do {
  <var> <- <expr>; -- bind
  let <fundef>; -- pure let
  <expr>; -- monadic expression
};

In the future we can consider some extensions:

1. Allow patterns of single constructor types on the left of <-.
2. @paulcadman mentioned Idris !-notation.
3. Do notation for Applicative.