Re-add MD4 support, if not at least for reading existing .MD4 checksum files?

[NintendoManiac64]

For whatever reason, on my dual-core Haswell CPU, I measured MD4 to actually be the fastest... (yes, faster than even CRC32).

So for many years I've been using MD4 with the original HashCheck Shell Extension and it'd be nice if I could still access those .MD4 checksums files. I'm not that concerned with the ability to create new MD4 files as I've generally moved on to SHA1 for its more widespread compatibility, but at least being able to read existing .MD4 checksum files would be nice (this is especially true since, now that I'm primarily a Linux user, I can't even use HashCheck Shell Extension at this time for creating checksums)

[NintendoManiac64]

I recently revisited the performance of this and I can definitely confirm that MD4 is the fastest algorithm when hashing the same set of 48GB of multiple data files, though this may be due to the oddity that CRC32 is slower than both MD5 and SHA1.

Unfortunately, because there's no version with both MD4 and BLAKE3, I cannot directly compare their results, especially since running v2.5.0 with only a single core result is quite a bit slower than running the old single-threaded-only versions (or at least the second-to-last single-threaded version of v2.2.3) - see issue #29

But looking at my results, MD4 is 20% faster than MD5 in v2.2.3 on the Athlon II x2 250, so considering that BLAKE3 isn't even 1% faster than MD5 in v2.5.0, it's quite likely that MD4 would still be faster than BLAKE3 (though it's also possible that, on any semi-modern CPU, you'd just end up I/O limited with any of these algorithms that aren't like SHA512 or whatever).

　
All systems were configured to 2core/2thread with 4x DDR3-1333 in dual-channel reading the 48GB of data from an SK Hynix Gold S31 500GB connected to a motherboard supporting 6Gbps SATA, and the resulting checksum file was written to an uncompressed ramdisk.

[v2.2.3] Athlon II x2 250

133 sec - MD4　◀ ◀
149 sec - MD5
160 sec - CRC32
181 sec - SHA1
464 sec - SHA256

[v2.2.3] Xeon E3-1246 v3 (basically a Xeon-badged i7-4790 non-K)

111 sec - MD4　◀ ◀
125 sec - SHA1
127 sec - MD5
138 sec - CRC32
325 sec - SHA256

[v2.5.0] Athlon II x2 250

115 sec - BLAKE3
116 sec - MD5
121 sec - SHA1
123 sec - CRC32
166 sec - SHA3-256
187 sec - SHA256

[v2.5.0] Xeon E3-1246 v3 (basically a Xeon-badged i7-4790 non-K)

100 sec - BLAKE3／MD5／SHA1 (I/O limited)
107 sec - CRC32
109 sec - SHA256
120 sec - SHA3-256