Add AsRef and Borrow for generate_mut_trait_impl

[A4-Tacks]

• AsRef -> AsMut
• Borrow -> BorrowMut

Example

//- minicore: as_ref
struct Foo(i32);

impl<T> core::convert::AsRef$0<i32> for Foo {
    fn as_ref(&self) -> &i32 {
        &self.0
    }
}

->

struct Foo(i32);

$0impl<T> core::convert::AsMut<i32> for Foo {
    fn as_mut(&mut self) -> &mut i32 {
        &self.0
    }
}

impl<T> core::convert::AsRef<i32> for Foo {
    fn as_ref(&self) -> &i32 {
        &self.0
    }
}

[ShoyuVanilla]

I think this should be receive hir::Trait though it's currently using &str before this PR.
Like here:

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;
	}

[ShoyuVanilla]

Those tests have redundant type parameter T