Simplify TapConfig.

[reitermarkus]

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Only allow Symbols for keys and booleans as values.

[Bo98]

I agree in principle, though this is public API which usually warrants a deprecation cycle. Have we checked nothing else anywhere is using it?

[reitermarkus]

Have we checked nothing else anywhere is using it?

It seems to be only used in tests and in Tap itself.

[Bo98]

Am OK with this then

[MikeMcQuaid]

Looks good, thanks again @reitermarkus! Please merge when you're around to handle any regressions.