erasure-coding: do not panic on 1 validator (#2022)

* erasure-coding: do not panic on 1 validator

* tabify fuzz tests

* remove num_validators length check

* remove unused import

* move erasure_coding_fuzzer to fuzzer

* change the authors to admit (at) parity.io

* Apply suggestions from code review

Co-authored-by: Sergei Shulepov <[email protected]>

Co-authored-by: Sergei Shulepov <[email protected]>