https://jira.mongodb.org/browse/BF-33577 highlighted that the bsoncolumn builder is only exercised by fuzzers indirectly through the fuzzers that exercise bsoncolumn decoding.  As a result, it's difficult for the fuzzer to fully explore the space of possible inputs to the bsoncolumn builder itself, and some bugs may be unseen for a long time.  We should add a fuzzer that directly exercises the builder and checks that it does not cause fatal errors for any inputs.  This requires devising a mapping from random fuzzer strings to the space of valid BSONElement inputs to the builder.