feat: Add `#[must_use]` to validation result enums

[alexanderwiederin]

BlockCheckResult, ProcessBlockResult, and ProcessBlockHeaderResult are all validation results that callers should always inspect.

All three should carry a #[must_use] attribute with a short message:

#[must_use = "check result must be inspected to know if the block is valid"]
pub enum BlockCheckResult { ... }

#[must_use = "placeholder"]
pub enum ProcessBlockResult { ... }

#[must_use = "placeholder"]
pub enum ProcessBlockHeaderResult { ... }