[WIP] SIMD acceleration

[chessai]

Adds SIMD acceleration via libbase64.
Depends on the yet-to-be-released haskell bindings library, libbase64-bindings.

Adds a cabal flag, simd, to enable SIMD. Enabled by default.

Benchmarks

I'm running this on a relatively quiet box, with NixOS/Linux installed.

CPU Info of the machine where I'm running these benchmarks:

❯ cat /proc/cpuinfo
...truncated...
processor	: 7
vendor_id	: GenuineIntel
cpu family	: 6
model		: 158
model name	: Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
stepping	: 9
microcode	: 0x48
cpu MHz        : 3473.454
cache size	: 6144 KB
...truncated...

❯ cat /proc/cpuinfo | egrep -o '(avx|avx2|ssse3|sse4_1|sse4_2)' | sort -u
avx
avx2
sse4_1
sse4_2
ssse3

If you download the current criterion benchmark reports you can view them in your browser.

For small ByteStrings, there is a significant penalty, so we should probably guard which encoding method is done based on the size of the ByteString. (EDIT: From experiments I determined that 1000 bytes is a safe threshold for encoding fallback, and 250 bytes for decoding. This PR implements the thresholds for encodeBase64_ and decodeBase64_, currently.)

Speedup summary tables (does not include unaffected under-threshold ByteStrings)

encodeBase64_ Speedups:

Size	vs old base64	vs base64-bytestring
1k	1.17x	3.07x
10k	3.45x	12.15x
100k	4.53x	11.71x
100mm	4.69x	14.21x

decodeBase64_ Speedups:

Size	vs old base64	vs base64-bytestring
1k	4.06x	5.01x
10k	11.29x	12.28x
100k	15.63x	16.44x
100mm	13.87x	19.48x

[chessai]

For encode, it seems like a reasonable threshold for SIMD to kick in is 1000 bytes.
For decode, it's looking more like 250 bytes.

[emilypi]

Let's get CI running and I"ll merge

[emilypi]

@chessai any movement on this? I'm trying to decide if i want to cut a release with or without SIMD support right now