Increase precedence of `=>` to match that of `for`.

Now

    for a => b => c

means

    for a => (b => c)
Restrict `for` to simple `a in b` expressions.
Rewrite import hook stuff.
Split special link handling into a separate function.

Also, `a.for x` will not be treated as a special construct anymore.
Add full `with`.

Syntax:

    with manager => something
    with var = manager => something
Fix an error during class creation if there are no constants.