fix: mod reduce msg_hash as input of the HMAC function

[obatirou]

What was wrong?

Eth-keys did not fully respect RFC6979 3.2.d.
The message digest should be modulo reduced before being passed as input of HMAC function.
This issue was found after this analysis.

According to the RFC, it should pass through a bits2octets function:

 K = HMAC_K(V || 0x01 || int2octets(x) || bits2octets(h1))

By looking into the definition of bits2octets, it states that the message digest needs to be reduced:

2.3.4.  Bit String to Octet String

   The bits2octets transform takes as input a sequence of blen bits and
   outputs a sequence of rlen bits.  It consists of the following steps:

   1.  The input sequence b is converted into an integer value z1
       through the bits2int transform:

          z1 = bits2int(b)

   2.  z1 is reduced modulo q, yielding z2 (an integer between 0 and
       q-1, inclusive):

          z2 = z1 mod q
   ...

Closes #101

How was it fixed?

Todo:

• Clean up commit history
• Add or update documentation related to these changes
• Add entry to the release notes

[obatirou]

Test are failing for coincurve < 17: those versions use libsecp256k1 prior to commit d8a246324650c3df8d54d133a8ac3c1b857a7a4e which do not include the PR of the fix for the modulo reduction bitcoin-core/secp256k1#1064

ofek/coincurve@v16.0.0...v17.0.0