proc-macro-srv's `TokenStream` needs to be backed by a `Rope` data structure

[Veykril]

Concatinating token streams is a very common operation for proc-macros. In r-a, this is currently very expensive due to it just cloning the data into a new contiguous chunk which can result in super slow proc-macro expansions. We should change this to use a rope instead making that operation effectively free.

[ChayimFriedman2]

That's really an interesting idea! Two thoughts::

1. Try it in rustc as well? Although their proc macro expansion is same-process and integrated with parsing.
2. We really don't have the same representation for both the server and the client!

[Veykril]

1. rustc already uses ropes for these everywhere.
2. We don't which is good! rust-analyzer's repr is optimized for memory usage, but the proc-macro side wants concat optimization so we actually want these impls to differ (and we can easily make this work thus)

[ChayimFriedman2]

Note that we don't really need a full rope (a balanced tree with accumulated length etc.), just a way to cheaply concatenate two token streams.