Prepare for the removal of OnLevelInit landing in stable

[nosoop]

See alliedmodders/sourcemod#1534.

I did a sprint to write up Entity Lump Manager. The best-case scenario is that it gets upstreamed and merged into core proper; otherwise it'll still exist as its own extension.

Because of some poor design decisions on my part, migrating Level KeyValues to use this implementation will mean copying the contents of the entity lump to StringMultiMap handles, since plugins have direct access to it. That'll probably have a performance impact, but dependencies shouldn't notice a difference otherwise.

The other option would be to just drop support for Level KeyValues entirely and force migration to the new natives, but that's no good either.

[nosoop]

Also went on a sprint to port LevelKeyValues to use the new extension. It's now available in the entitylumpext branch.

Even with copying between the extension's EntityLump to StringMultiMap handles, the process from parsing the original entity string to dumping the modified one is only 20% slower compared to raw char[] parsing on a large-ish TF2 map on Linux (cp_snowplow).

While I do encourage migration to using the Entity Lump Manager API directly (it should remain faster), Level KeyValues will continue to work with SM 1.11 as long as it's up to date with the dependency on the new API.