Add a new trait `proc_macro::ToTokens`
Tracking issue #130977
This PR adds a new trait `ToTokens`, implemented for types that can be interpolated inside a `quote!` invocation.
```rust
impl ToTokens for TokenTree
impl ToTokens for TokenStream
impl ToTokens for Literal
impl ToTokens for Ident
impl ToTokens for Punct
impl ToTokens for Group
impl<T: ToTokens + ?Sized> ToTokens for &T
impl<T: ToTokens + ?Sized> ToTokens for &mut T
impl<T: ToTokens + ?Sized> ToTokens for Box<T>
impl<T: ToTokens + ?Sized> ToTokens for Rc<T>
impl<T: ToTokens + ToOwned + ?Sized> ToTokens for Cow<'_, T>
impl<T: ToTokens> ToTokens for Option<T>
impl ToTokens for u{8,16,32,64,128}
impl ToTokens for i{8,16,32,64,128}
impl ToTokens for f{32,64}
impl ToTokens for {u,i}size
impl ToTokens for bool
impl ToTokens for char
impl ToTokens for str
impl ToTokens for String
impl ToTokens for CStr
impl ToTokens for CString
```
~This PR also implements the migration mentioned in the tracking issue, replacing `Extend<Token{Tree,Stream}>` with `Extend<T: ToTokens>`, and replacing `FromIterator<Token{Tree,Stream}>` with `FromIterator<T: ToTokens>`.~
**UPDATE**: Reverted.
```diff
-impl FromIterator<TokenTree> for TokenStream
-impl FromIterator<TokenStream> for TokenStream
+impl<T: ToTokens> FromIterator<T> for TokenStream
-impl Extend<TokenTree> for TokenStream
-impl Extend<TokenStream> for TokenStream
+impl<T: ToTokens> Extend<T> for TokenStream
```
I'm going to leave some comments in the review where I'm unsure and concerned.
r? ``@dtolnay``
CC ``@tgross35``
c9347f1e27