Removal of 'Domain'

With v0.5.0, the old Domain class is removed in favor of Symbol for domain identifiers in the re-entrant mutex.

Symbols have the benefit of being allowed in plain objects. Symbols also have additional convenience methods built-in.

This change also reduces the bundle size ever so slightly since it is missing the Domain class definition.

Typing Updates to Keyed Mutex

• Simplified key resolver function type signature, generic can be any type.
• By default, the KeyedMutex will accept string | number | symbol keys, instead of defaulting to string. This can automatically be narrowed by providing a type-annotated resolver function.

Remove LockType enum from RWMutex

• Removed the LockType enum from the RWMutex
• typescript enums cause issues with tree-shaking
• Moved to string union type, and renamed to RWLockType
• Package can now be marked side-effect free