Add new syntax for `if` and `while`; break `for`.

    if condition 1 do action 1
       condition 2 do action 2
       otherwise   do action 3

    for x in xs do something  # DOES NOT WORK YET

    while condition do something
Simplify parsing of if's.

Reading everything up to the end of the line is enough.
Add some variety to the `unmatched block start` error message.

The REPL should correctly recognize open parentheses again.
Change the name of the second argument of `raise`.

    raise KeyError cause: ValueError
Allow to fit an entire `if` on one line.

    if (condition do action) (condition do action) ...
Abuse features from the latest 2 commits immediately.
Replace `do` with `=>`.

   if a => b ...
Make `unsafe` (now `except`) match `if` in syntax.

Complete with a new keyword.

    except err => do_something_ridiculous
           err :: KeyError => print 'You got a KeyError, wow'
           err :: TypeError => raise ValueError
           finally => system $ 'rm ' + __file__