Tune base64 decoding

This improves performance of base64-decode-region by about 7.5% on
my platform, and gets rid of some macros.
(base64_value_to_char, base64_char_to_value):
Now an array of two arrays.  All uses changed.
(base64url_value_to_char, base64url_char_to_value):
Remove.  All uses changed to the other array.
(base64_char_to_value): Entries are now of type signed char, not
short, since we can assume C99.  Use C99 initializers; this is
clearer and caters to the (theoretical) possibility of systems
that do not use ASCII or do not have 8-bit bytes.  Allow any index
in the range 0..UCHAR_MAX instead of limiting it to 0..127, so
that uses need not check for in-range indexes.  Also record
padding chars.  All uses changed.
(base64_decode_1): Always store number of chars in *NCHARS_RETURN,
for simplicity.  All callers changed.  Speed up the byte-fetching.
1 job for master in 7 seconds (queued for 4 seconds)
Status Job ID Name Coverage
failed #2052


Name Stage Failure
test-all Test There has been a runner system failure, please try again
Running with gitlab-runner 11.6.0 (f100a208)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:8d31923452f8b79ae91b01568d28c90e7d667a9eaff9734c6faeb017b0efa8d0 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: failed to start shim: exec: "docker-containerd-shim": executable file not found in $PATH: unknown (executor_docker.go:1018:1s)