Improve performance of String names

A common source of performance problems in JuMP and MathOptInterface are dealing with the `String` names of variables and constraints.

As a recent example, see https://github.com/jump-dev/MathOptInterface.jl/pull/2426 and related issues/PRs.

The main place that `String`s are stored is:

https://github.com/jump-dev/MathOptInterface.jl/blob/dcfb03311bb6afc10a36e7967cdd9aecb706241c/src/Utilities/model.jl#L27-L31

When building models with large (>10^5) numbers of variables and constraints, these dictionaries contain a large number of strings. Each `String` is a GC-tracked object, and it is long-lived, because we maintain these dictionaries throughout the lifetime of a `Model`. If `CachingOptimizer` is involved, there can also be multiple copies of the strings, for example, if we have a setup like `Cache(Model, Bridge(Cache(Model, Solver)))`.

Lots of long-lived GC-tracked objects are bad for Julia's garbage collector.

## Workaround

The main workaround at present is `JuMP.set_string_names_on_creation(model, false)`, which does not pass string names from JuMP to the solver.

The downside of this workaround is that solvers and file writers cannot print or display useful names to the user.

* https://github.com/jump-dev/JuMP.jl/issues/2973
* https://github.com/jump-dev/JuMP.jl/issues/2817

## Potential approaches

We should try replacing the `String` storage with an alternative string type. Candidates are:

* https://github.com/JuliaStrings/InlineStrings.jl
* https://github.com/JuliaData/WeakRefStrings.jl

To ensure backwards compatibility, we should make sure that getting and setting these strings uses the original `String` type, so that the internal storage is hidden from the user.

It is okay to convert between string types, and to generate extra allocations if these new strings are short-lived objects. The real performance problem is long-lived GC-tracked objects.

## Solvers

Many solver packages also store `String` names. If the changes to MOI are an improvement, we should investigate was to simplify name handling in the different solver wrappers.