optimize idPool() a bit

[WalterBright]

You see... a while back, I was walking down the yellow brick road and noticed that the idPool function had remnants of the old C version of the implementation. Thought I'd fix it by:

1. removing the ptr/length interface, except for the C++ entry point
2. use a default value instead of a forwarding function
3. recognize that one of the overloads was only used for keyword setup
4. remove 32 bit legacy cast to uint
5. add some clarifying documentation

[dlang-bot]

Thanks for your pull request, @WalterBright!

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