New method system

[dominik-muc]

Instead of current approach, we could see methods on types as a kind of feature extension.
Let's separate them from the type declaration, because:

• it allows to extend external types (for example coloring methods for strings)
• it can group similar behavior
• it allows for parameter-specific extensions (for example .product() on Vec<i32>)

The main issue that many languages, including Rust, suffers from, is that two independent
crates can implement the same methods/traits, resulting in conflicts. Rust prohibits this
by coherence rules, but I have a (maybe?) better idea — let the extensions be named.
A method is accessible if the name is imported by scope. If two, differently named
extensions add method with the same name, disambiguation is requested by the compiler.

I propose the following syntax:

ext <name> for <type> {
    <methods>
}

The question is whether to allow generic extensions (eg. ext<T> vec_impl for Vec<T>)