Add ide-assist; Generate AsRef impl from Borrow

[A4-Tacks]

Generate AsRef implement from Borrow.

struct Foo<T>(T);

impl<T> $0Borrow<T> for Foo<T> {
    fn borrow(&self) -> &T {
        &self.0
    }
}

->

struct Foo<T>(T);

$0impl<T> AsRef<T> for Foo<T> {
    fn as_ref(&self) -> &T {
        &self.0
    }
}

impl<T> Borrow<T> for Foo<T> {
    fn borrow(&self) -> &T {
        &self.0
    }
}

[ShoyuVanilla]

As this is a new assist and relatively simple, could this be implemented with new SyntaxEditor infra?

[ShoyuVanilla]

This would be better semantically like:

rust-analyzer/crates/ide-assists/src/handlers/convert_into_to_from.rs

Lines 35 to 44 in 5b852da

	let impl_ = ctx.find_node_at_offset::<ast::Impl>()?;
	let src_type = impl_.self_ty()?;
	let ast_trait = impl_.trait_()?;
	let module = ctx.sema.scope(impl_.syntax())?.module();
	let trait_ = resolve_target_trait(&ctx.sema, &impl_)?;
	if trait_ != FamousDefs(&ctx.sema, module.krate()).core_convert_Into()? {
	return None;
	}