Batch signature verification (#5023)

* create parallel tasks extension

* make type system happy

* basic externalities

* test for dynamic extensions

* batching test

* remove premature verify_batch

* shnschnorrkel batch

* alter test

* shnschnorrkel test

* executive batching

* some docs

* also multi/any signatgures

* error propagation

* styling

* make verification extension optional

* experimental ed25519 parallelization

* some merge fallout

* utilize task executor

* merge fallout

* utilize task executor more

* another merge fallout

* feature-gate sp-io

* arrange toml

* fix no-std

* sr25519 batching and refactoring

* add docs

* fix name

* add newline

* fix block import test

* long sr25519 test

* blocking instead of parking

* move everything in crypto

* return batch_verify to check :)

* use condvars

* use multi-threaded executor for benches

* don't call via host interface

* try no spawning

* add true...