This updates the SourcePawn library, which breaks binary compatibility for extensions (not for plugins) due to many changes to the public API.

The most significant change is that ICallable is removed. Instead, arguments are now packed into a CallArgs structure which is then passed to the VM. CallArgs is much more efficient than the old ICallable::Push API, reducing invocation overhead by about 15% on my computer. functions.inc now uses CallArgs internally, and IForwardSys has been augmented to use CallArgs as well.

Extensions do need to be recompiled. However, the old "Push" API is retained for source-level compatibility, which is why almost no first-party extensions were changed in this PR. Well-behaving extensions will recompile with no issues.

Some extremely esoteric features were removed, so extensions using them will no longer work.

• IForwardSys no longer supports variadic functions. This feature had no use in the corpus or internally, and was almost impossible to actually use in any meaningful way.
• An IPluginContext function specifically to make VFormat() work is now restricted to core.
• Any function in sp_vm_api.h that was not used by SourceMod's first-party extensions has been removed from the public API. These should generally have never been used, and were mostly related to native binding or named function lookup (which should be done via ForwardSys or GetFunctionById).