feat: reduce the amount of heck-related allocations

[vrmiguel]

What kind of change does this PR introduce?

• Bugfix
• Feature
• Docs
• New Binding issue #___
• Code style update
• Refactor
• Build-related changes
• Other, please describe: Super slight performance improvements

Does this PR introduce a breaking change?

• Yes, and the changes were approved in issue #___
• No

Checklist

• When resolving issues, they are referenced in the PR's title (e.g fix: remove a typo, closes #___, #___)
• A change file is added if any packages will require a version bump due to this PR per the instructions in the readme.
• I have added a convincing reason for adding this feature, if necessary

Other information

Hi! This small PR avoids heap allocations caused by casing conversions, whenever possible.

Example:

// Current code
// Three Strings are allocated below, two of them immediately freed after
format!(
    "CARGO_FEATURE_{}",
    feature
         .to_snake_case()
         .to_uppercase()
  )

// Proposed code
// Only the string returned by `format` is allocated
format!("CARGO_FEATURE_{}", AsShoutySnakeCase(feature))
// AsShoutySnakeCase is a struct which performs the conversion in its Display implementation