Use base64 library by @aklomp

[k0ekk0ek]

Use a slightly modified version of @aklomp's generic base64 decoder. While implementing support for ech on #16 (not in RFC, but supported by NSD) I required yet another slightly modified base64 parse function. Instead I opted to pull in the excellent stream decoder. While not leveraging the vectorized implementations yet (#27), using the decoder boosts performance significantly, especially for DNSSEC heavy zones (i.e. .se).

before:

time ./zone-bench -t haswell parse ../../zones/se.zone
Selected target haswell
Parsed 8924051 records

real    0m2.832s
user    0m2.511s
sys     0m0.283s

after:

time ./zone-bench -t haswell parse ../../zones/se.zone
Selected target haswell
Parsed 8924051 records

real    0m1.817s
user    0m1.390s
sys     0m0.274s

I've also added an endian.h that pulls in the system's endian.h if available (not for macOS and Windows). The byteswap routines can be used to avoid use of hton* and ntoh* functions so we don't depend on winsock.h on Microsoft Windows.