remove calls to `__instancecheck__` on `ModelValidator`

[samuelcolvin]

As of #492 ModelValidator is checking if the input is an instance of BaseModel before doing anything else.

Because BaseModel is an abc, the isinstance rust call is eventually calling the python __instancecheck__ method on BaseModel's metatype.

isinstance checks have significant performance problems, see pydantic/pydantic#3829 and python/cpython#92810.

We should look at removing this check by implementing a faster isinstance check but just looking at the type, and the type's tp_base (or maybe tb_bases) - this would effectively lead to a fast_isinstance method.

[samuelcolvin]

Some initial benchmarks indicate this isn't a major issue, although it could lead to memory growth as per python/cpython#92810.

We should look at this before releasing v2.

[romulocollopy]

@samuelcolvin is this a good first issue? I'm eager to practice my Rust in real life

[samuelcolvin]

Probably not, I've tried and this isn't making a big difference to performance.

Honestly their probably aren't that many good issues if you want to learn rust right now I'm afraid.

[samuelcolvin]

you could try looking at #584, copy that code and create a new PR?