Implements a new method to generate instance names for anonymous
instances that uses a heuristic that tends to produce shorter names. A
design goal is to make them relatively unique within projects and
definitely unique across projects, while also using accessible names so
that they can be referred to as needed, both in Lean code and in
discussions.
The new method also takes into account binders provided to the instance,
and it adds project-based suffixes. Despite this, a median new name is
73% its original auto-generated length. (Compare: [old generated
names](https://gist.github.com/kmill/b72bb43f5b01dafef41eb1d2e57a8237)
and [new generated
names](https://gist.github.com/kmill/393acc82e7a8d67fc7387829f4ed547e).)
Some notes:
* The naming is sensitive to what is explicitly provided as a binder vs
what is provided via a `variable`. It does not make use of `variable`s
since, when names are generated, it is not yet known which variables are
used in the body of the instance.
* If the instance name refers to declarations in the current "project"
(given by the root module), then it does not add a suffix. Otherwise, it
adds the project name as a suffix to protect against cross-project
collisions.
* `set_option trace.Elab.instance.mkInstanceName true` can be used to
see what name the auto-generator would give, even if the instance
already has an explicit name.
There were a number of instances that were referred to explicitly in
meta code, and these have been given explicit names.
Removes the unused `Lean.Elab.mkFreshInstanceName` along with the
Command state's `nextInstIdx`.
Fixes#2343
