Allow implementing methods without impl blocks

[osa1]

Consider the Str methods:

impl Str:
    fn fromUtf8Vec(bytes: Vec[U8]): Str = ...

    fn len(self): U32 = ...

    fn isEmpty(self): Bool = ...

    fn substr(self, byteStart: U32, byteEnd: U32): Str = ...

    fn startsWith(self, prefix: Str): Bool = ...

We should allow implementing these without an impl block:

fn Str.fromUtf8Vec(bytes: Vec[U8]): Str = ...

fn Str.len(self): U32 = ...

fn Str.isEmpty(self): Bool = ...

fn Str.substr(self, byteStart: U32, byteEnd: U32): Str = ...

fn Str.startsWith(self, prefix: Str): Bool = ...

Now sure how to best handle types with type parameters:

type Vec[T]: ...

impl[T] Vec[T]:
    fn withCapacity(cap: U32): Vec[T] = ...

Some options:

fn Vec[A].withCapacity(cap: U32): Vec[A] = ...  # A matches the type parameter in `type Vec[T]`

fn Vec.withCapacity(cap: U32): Vec[T] = ...     # T inherited from the type definition