Adding user-definable context variable guards

[BrodyHiggerson]

Fitting with the "don't pay for what you don't use" concept, I wanted to have an optional way of injecting some kind of lock/guard around context variables in the registry.

I let the user define how the lock is created since, for me, I use a fiber mutex rather than std::mutex, so I need to be able to define it myself. I'm using the lock in the 'leaf nodes' of the function calls since I can't support recursive mutexes and the end result is the same.

(I'm making this issue since the contribution guidelines don't want immediate PRs. The commit I would PR is here: BrodyHiggerson@8870d62)

How I'm using it:

namespace ftl
{
class Fibtex;
} // namespace ftl

namespace detail
{
ftl::Fibtex& GetContextVarFibtex();
} // namespace detail

#ifndef ENTT_REGISTRY_CONTEXT_GUARD
	#define ENTT_REGISTRY_CONTEXT_GUARD()										\
		ftl::LockWrapper wrapper = rtg::Task::LockFibtex(detail::GetContextVarFibtex());\
		std::lock_guard guard(wrapper);
#endif // #ifndef ENTT_REGISTRY_CONTEXT_GUARD

[skypjack]

Closing this one, let's continue the discussion in #800 (that is, the PR).👍