Add implicit conversions between Ulid and string, small performance optimizations, update README and tests accordingly.

[Seramis]

Description

Implemented ToString() with no arguments for better compatibility. Previous ToString(string? format, IFormatProvider? formatProvider) just calls the ToString() without arguments.

Added implicit conversions between Ulid and string.

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):

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.