Xlfunctions

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- Nothing changed yet.

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- Implement a few more operators for the Excel Number type.

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- Fix an error message to refer to the righ type.

- Added a test to ensure SUM() works with arrays containing Excel data types.

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- Support for delayed argument execution by introducing expressions that can
be evaluated when needed. This is required to support efficient logical
operator implementations. For example, when an "if"-condition is true, the
false value does not need to be computed.

- Implemented all Excel types.

+ Better control of logic differences between Python and Excel. (Compare
with None and blank handling, for example.)

+ Tight control of type casting with very situation-specific edge case
handling. (For example, when a string representing a boolean will evaluate
as a boolean and when not. For example, `int(bool('False')) == 0` in Excel
but `AND('False', True) == True`.

+ Make date/time its own type.

- Moved errors back into their own module.

- Moved criteria parsing into its own module.

- Made function signature validation and conversion much more consistent
allowing a lot less error handling and data conversion in the function
body.

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- Complete rewrite of library.

* Introduced a function registry that can be used to extend the function
library in third party software.

* Removed excessive use of static methods and converted all Excel functions
to simple Python functions (with some decorators).

* Organized functions into categories based on Microsoft documentation.

* Proper argument validation and conversion where supported.

* Many functions are now much more flexible with their types and more
correctly mimic Excel behavior.

* Use of `dateutil` and `yearfrac` libraries to do complicated date
calculations instead of implementing it from scratch.

* Achieved 100% test coverage.

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- Initial release.