Refactor base64 decoding to use a lookup table #590

Job	Run time
custom-install-linux	9s
custom-install-win	3m 16s
default-linux	7s
default-linux-with-examples	7s
default-win	3m 5s
min-req	9m 5s
root-hint-install-linux	11s
preset-support-but-not-enough	12m 9s
no-base64	13s
no-pico	10s
with-hunter	3m 12s
with-libressl	1m 25s
with-wolfssl	1m 0s
	34m 9s

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