Fixed error handling, removed some typos, and optimized performance by little bit

[0x0Glitch]

This PR addresses three separate issues:

1. Fixed typos in error messages:

   • Changed "cant" to "can't"
   • Fixed "form a stateless view" to "from a stateless view"
   • Fixed "statless" to "stateless"

2. improved error handling:

   • Added missing sentinel error variables to doc.go
   • Modified implementations to use these central error variables
   • Enables proper error checking with errors.Is() throughout the codebase

3. Fixed O(N²) DoS vulnerability in proof_ipa.go:

   • Added keyStemIndex map for efficient lookups
   • Replaced linear scans with map lookups in 3 locations for optimizing performance
   • Changed performance from O(N²) to O(N) for large proofs(it will prevent DOS)

[0x0Glitch]

Hello @gballet,please have a look at the third point(Fixed O(N²) DoS vulnerability in proof_ipa.go) if that optimizes the performance