Add type assertion syntax for runtime type checking.

[profan]

Right now, I assert that a lot of things are numbers, tables, non-null etc in function bodies, but it would be nice if we could automate these asserts with some nice syntax.

function(x, y, z)
    assert(x ~= nil)
    assert(y ~= nil)
    assert(z ~= nil)
end

would become

function(x?, y?, z?)
end

where the compiler would then insert the code we'd have written by hand normally.