Add Base45 decoding

Volker Krause requested to merge work/base45-decode into master

Base45 (sic!) is a codec sometimes used for putting binary content into QR barcodes, as it's output character set matches what QR can encode using its second most efficient symbol set.

A practical example are the digital vaccination certifications currently being deployed in Europe.

The code has been put into a separate file to allow for a more future-proof licensing than what kcodecs.cpp uses.

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