Moving stdlib as repository

The needs are specific, just to move the discussions in another place, so we can focus in this repo in the compiler itself (and builtins).

The flow:

* [x] Wait for 0.3.5 release
* [x] Merge all the pending PRs for stdlib stuff
* [x] Create a new repo, https://github.com/amber-lang/amber-stdlib
* [ ] move them (to define if we can move also the git history or we don't care), remove from this repository (together with tests)
* [ ] Create in this repository the stdlib as gitmodule so we can block it to a specific commit, symlinked to the actual folders
* [ ] Update the CI and the readme