Performance improvements on new Ulid generation and Parse()/ToString()

[Seramis]

Description

Improved code path generating new Ulids. Using ref pointers instead of Span<> to avoid unnecessary bounds checking.
Similar improvements done to ToString() and Parse() code paths.
README updated with fresh benchmark data.
Nugets updated.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Other (please specify): Performance improvements

Checklist

• The PR is submitted to the correct branch (main).
• My code follows the project's coding style. (.editorconfig)
• I have commented my code, particularly in hard-to-understand areas and public interfaces.
• I have added or updated tests for the changes I made.
• All new and existing tests passed.
• I have updated the documentation where applicable.