[marchc1]

This fixes a couple of the largest issues I could find with initialization:

• SoundEmitterSystem using LinkedList.Find to find a node; this replaces those operations with a dictionary lookup (was nearly 25% of startup!!)
• VPK indices don't read one byte at a time now
• VPK's aren't read twice (another packstore can reuse the same vpk data)
• FilenameAndExtension is lazy initialized
• Overall, VPK mounting should be ~4x faster
• FindExcludeProp was checking all slots, not just filled in ones, accidentally missed that (nearly 10x faster I think)

A lot of the remaining overhead is likely JIT and other factors, of which I will continue to look into later, but I'm sure there's some other very silly things I'm doing that need to be checked as well. For later:

ConVar_Register is probably the biggest one since it does a huge amount of reflection. [ConCommandAttribute] could probably be turned into a code generator. I am unsure if source generators could work without attributes on the convars, but perhaps we could even compile those rather than relying on reflection.

The field accessors are also pretty meh
PostBuildAllForms is also very reflection heavy
may also want to look into reworking the file find API