Refactor type fn for readability/brevity

This replaces the helper function `var type = {}.toString` with `var type = function(obj) {return {}.toString.call(obj)}`.
This allows it to be called more ergonomically.

It replaces invocations (previously, `type.call(obj)`) with `type(obj)`.

Incidentally, it reduces mithril.js by 7 bytes

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All tests still pass, as run with `grunt`'s default.