databases/puredb: Set of libraries for creating and reading constant databases

PureDB is a portable and tiny set of libraries for creating
and reading constant databases. It manages data files that
contains text or binary key/data pairs of arbitrary sizes.
Lookups are very fast (normally only one disk access to
match a hash value), overhead is low (a database is 1028
bytes plus only 16 extra bytes per record), multiple
concurrent read access are supported, and databases can be
up to 4 Gb long, and they are portable across architectures.

PR: ports/48901
Submitted by: Sergei Kolobov <sergei@kolobov.com>