Exphash from sha256 to md5 to match imphash

[N0fix]

PR #354 added exphash using the sha256 function instead of md5, which creates two issues:

• There is a lack of consistency between imphash (which uses md5) and exphash (currently using sha256).
• The associated YARA implementation (ongoing PR) uses md5.

This PR proposes using md5 instead of sha256 for exphash to maintain consistency.

Note: I believe this confusion stems from a discrepancy between the original blog post about exphash, which mentions the usage of sha256, and its integration into public community projects.

[erocarrera]

I'm a bit torn in this case: I see the rationale in staying consistent with imphash, even more in the case that yara is also using md5.
Ideally all these methods would be using sha256, but one can't update imphash without causing major issues, I'd say it's not worth the pain.
Hence I'm leaning towards merging and using md5 and hoping not many people have started relying on the sha256 results.