firebase-ios-sdk

Migrate [LevelDBKey descriptionForKey:] to DescribeKey() (#1638)

* Convert FSTLevelDBKeyTests to DescribeKey()

Note that the description coverage is already in leveldb_key_test.cc

* Rename Describe to DescribeKey

* Fix pointer alignment in leveldb_key.cc

* Add DescribeKey overloads for other string types

Unforunately, both leveldb::Slice and absl::string_view implicitly
convert from std::string which makes providing overloads for both
ambiguous for anything that converts to either.

* Migrate [FSTLevelDBKey descriptionForKey:] to DescribeKey()