Add fuzzing tests

See for example: https://github.com/tendermint/tendermint/tree/master/test/fuzz

Review the linked Tendermint test/fuzz example first, then inspect rsmt2d's existing test entry points to determine which behaviors should be fuzzed. Done means adding appropriate fuzzing coverage to the repository and confirming the test suite runs successfully; the issue does not name specific local files or cases.

Written by the indexing model from the issue text.