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Allow global per-thread values to be defined within a certain scope in a way that supports multiple values, temporary changes inside with contexts, etc.

This is implemented in two layers. The base layer is a map from keys to values which isolates different, broad, functionalities. Despite the name, a NamespaceMap can map from any key to any value - it's just a thread-local map. However, typically is it used with Namespaces because they have support for some useful idioms.

A Namespace is, as described above, associated with a name in the thread's NamespaceMap. It manages state for some functionality, so is another map, forming the second layer. The motivating example of a Namespace is the OperatorNamespace, which maps from operators to matchers. This uses the support in Namespace that allows values to be over-ridden within a certain scope to support overriding matchers for matching spaces.