Remove IntoIn<'a, Atom<'a>> bounds from AstBuilder methods

[overlookmotel]

Many AstBuilder methods take an IntoIn<'a, Atom<'a>> where an Atom is required. This was intended to give flexibility, but it makes it too easy to accidentally copy existing Atoms and make duplicate copies of them in arena, when could just use the original Atom.

e.g. oxc-project/oxc#7969

Make AstBuilder methods just take Atom<'a>, and make user generate the Atom themselves. Usually they'll find they don't have to, because they have an existing Atom already.

[overlookmotel]

Related: #155.