Rewrite PropertyTable to use std::unordered_map

[adamkewley]

Reimplements PropertyTable to store the string-to-index lookup in an std::unordered_map, rather than a std::map.

The motivation for doing this is model editing performance. PropertyTable, Component, Socket, etc. are all using std::map, which is almost always implemented as a red-black tree. This means that copying a Model (e.g. because an editor wants to back up an old version) spends a large amount of time copying tree nodes, strings, etc.

Changing the lookup to an O(1) lookup also has the advantage of making runtime property lookups faster

I will try to build a downstream fork of OSC with this branch to get an idea of where the performance impact is, but I figure that it's a nice refactor to do in general.

Brief summary of changes

• Change PropertyTable to store PropertyHolders (pointers to AbstractProperty)
• Use modern C++ abstractions to remove the manual copy ctor/assignment etc.

Testing I've completed

• Ran the original test suite, which extensively tests the property API

Looking for feedback on...

CHANGELOG.md (choose one)

• no need to update because internal

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This change is 

[adamkewley]

Downstream benchmarking indicates that this doesn't have a big impact on model copying, so this is more of a feel-good change ;)

[adamkewley]

Thanks for checking it out @nickbianco - too bad it doesn't have any particularly big gains, but hopefully it's an otherwise nice little clean-up!

[nickbianco]

@adamkewley definitely worth a shot, and yes, a nice refactor regardless :)