Improve Tlsh library in Yara

[dmknght]

Is your feature request related to a problem? Please describe.
The current Tlsh code is the TLSH-C, which was ported by Avast dev. This version is missing 2 functions

1. Calculate the diff score of 2 Tlsh hashes (Add total_diff to tlsh.c and tlsh.h avast/tlshc#1)
2. Load the hash value from a string (Add fromTlshStr to the tlsh.h avast/tlshc#2)
   Therefore, Tlsh version that Yara is using can do hash matching only. IMO tlsh is a fuzzyhash algorithm so it should has score diffing

Describe the solution you'd like
In https://github.com/VirusTotal/yara/blob/master/libyara/tlshc/tlsh.c, Yara can add

int tlsh_total_diff(Tlsh* tlsh, Tlsh *other, bool len_diff)
{
    return tlsh_impl_total_diff(tlsh->impl, other->impl, len_diff);
}

int tlsh_from_tlsh_str(Tlsh* tlsh, const char *str)
{
    return tlsh_impl_from_tlsh_str(tlsh->impl, str);
}

And then add a function in ELF module to compare hash (maybe create an other function to compare with diff_len)

bool yr_elf_tlsh_cmp(char *hash, int user_score)
{
    Tlsh tlsh = new_tlsh();
    tlsh_from_tlsh_str(tlsh, hash);
   // Code block to get hash from metadata here
   if (user_score < tlsh_total_diff(tlsh, <elf_tlsh_struct>, False))
       return True;
   return False;
}