Add a new field to ovector_t objects to record real memory usage in bytes.

  The length field is used for the language semantics usage, but the
memory is always aligned to 16 bytes.
  The new field was also added to "derived" like objects, that use the
same vector allocator, ohash_t, ohashtable_t and ostream_t.
  This field is currently really only useful for ensuring language
string_t objects can be easily "coerced" to C "char *" strings, but
could have other special cases where the value of the real size in bytes
may be desirable to know.
Add base/initial SDL2_mixer support.