Sigils

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Added (simplified) support for function tools once again.

This should work: %[sigil-name.field.upper]

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Restore backwards compatibility with Python 3.9.

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Complete revamp and simplification of the library.

Sigils now use the following syntax:

%[sigil-name]
%[sigil-name:arg1:arg2:...]
%[sigil_name]
%[sigil_name:key1=value1:key2=value2:...]

Check the new README.rst for more information.

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- Refactored the FUNCTION_MAP into the *funcs.py* module.
- Added a simple benchmarking script.

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- Fixed a bug in the *unvanish* function.

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- Added pyyaml dependency.
- Added tox and githib actions for automated testing.