Decoder performance refactor

[arapov]

The current state of things:

The routine invoking the decoders first builds all the possible chains of decoders with the current providers, propquery and other parameters. This is cached so this expensive build step is performed just once for given set of parameters (unless providers are loaded/unloaded which invalidates the cache).

Then the decoder chains are iteratively and recursively called until something is decoded and the object construction succeeds.

The problems are:

• The decoder chain cache can be fairly big.
• Some hints in the data being decoded is lost (this needs to be verified but for example things like PEM headers indicating concrete key types are probably not considered).
• Some decoders that have no chance to succeed are still tried.
• Even with the chain cache added the decoders are still fairly slower (no longer order of magnitude but...) than what we had in 1.1.1.

Possible solutions:

• Prioritization of PKCS#8 der decoders might avoid many decoding dead-ends
• Make it possible to provide hints from sibling decoders to eliminate other useless decoding steps (I.e., if PKCS#8 decoder found that the DER data is PKCS#8 but for a key that it cannot handle, there is no point in invoking non-PKCS#8 der-to-key decoders.)
• Radical solution: Create one or two all encompassing decoders that just do PEM2KEY and DER2KEY conversions directly, prioritize them over the existing step-by-step decoders - basically bypassing the existing decoder architecture for keys that we know how to handle.
• Further ideas TBD

### Tasks
- [ ] https://github.com/openssl/project/issues/111
- [ ] Implement the proposed decoder perf. refactor

[krk]

Is there a concrete definition of what the new design should look like?

[mattcaswell]

No (but see #111)

[t8m]

I've updated the description to provide some useful starting points.

[t8m]

BTW the current state of the decoder performance in 3.3 is much better than it was in 3.0. It is of course still not as fast as 1.1.1 was but the performance degradation is no longer catastrophic.