Small source code optimizations. Clarified ULID advantages in README.md.

[Seramis]

Description

• Some readonly keywords were removed from methods that didn't need them.
• Moved the length check out from TryFill() method and renamed it to Fill(). Length check is done now when necessary before Fill() is called.
• Benchmarks were not affected.
• Added one additional unittest for method call of Ulid.New(timestamp, random);.
• Clarified ULID advantages over GUIDs, including UUIDv7 GUID, in README.md.

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): Codestyle

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.