Optimize the string allocations

[DamianEdwards]

The validator has to allocate strings for each field it validates, including adding a level prefix as it descends the object graph, even if there don't end up being any errors. Would be good to look at if there's a way to minimize these allocations.

Examples:

• https://github.com/DamianEdwards/MiniValidation/blob/7efcac2e2b82fd5c69f9be1a1b4587032afe9a2e/src/MiniValidation/MiniValidator.cs#L185
• https://github.com/DamianEdwards/MiniValidation/blob/7efcac2e2b82fd5c69f9be1a1b4587032afe9a2e/src/MiniValidation/MiniValidator.cs#L190
• https://github.com/DamianEdwards/MiniValidation/blob/7efcac2e2b82fd5c69f9be1a1b4587032afe9a2e/src/MiniValidation/MiniValidator.cs#L247
• https://github.com/DamianEdwards/MiniValidation/blob/7efcac2e2b82fd5c69f9be1a1b4587032afe9a2e/src/MiniValidation/MiniValidator.cs#L288
• https://github.com/DamianEdwards/MiniValidation/blob/7efcac2e2b82fd5c69f9be1a1b4587032afe9a2e/src/MiniValidation/MiniValidator.cs#L324