Fix BLS pop dst and no context API

[perturbing]

Description

In the IETF draft of BLS signatures, the domain separation tag of the proof of possession should be separated from normal signatures. E.g., this means we should use the prefix tag BLS_POP_ for the PoP and BLS_SIG_ for signatures (which we already did).

Before, we used the BLS_SIG_ prefix for PoP as well; this is insecure as you can trick a key holder into signing a PoP without that intent via the signature path(can be useful in a rogue key attack).

This PR also fixes a typo in the Curve family description.

Credits to @colll78

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages.
  New section is never added with the code changes. (See RELEASING.md)
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated.
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Self-reviewed the diff

[ch1bo]

Can we make the API simpler for the normal signature case too?

[colll78]

Identified and reported by Anastasia Labs.

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