test(fuzz): read -> write bson

[LeoDog896]

Runs to_writer in fuzz tests. Makes the fuzzer slower, but covers any write errors. Also adds a fuzz test for from_slice_utf8_lossy.

Note: I did not check for equivalence in from_reader and to_writer, as, even for accounting for length, there is still some loss in data ordering from the reader, as is to be expected.

[abr-egn]

Thank you, this is a very reasonable addition! I've tagged in Isabel for final approval.