Incompatibility with `heck` crate

[frozenlib]

The recognition of word boundaries when using style = "..." differs from that used in heck crate.

When HTTPClient is converted to snake case, it will appear as follows:

• parse_display: httpcleint
• heck: http_client

This is due to the lack of implementation of rule 3 of the heck crate's word boundary definition in parse_display:

1. All underscore characters are considered word boundaries.
2. If an uppercase character is followed by lowercase letters, a word boundary is considered to be just prior to that uppercase character.
3. If multiple uppercase characters are consecutive, they are considered to be within a single word, except that the last will be part of the next word if it is followed by lowercase characters (see rule 2).

I plan to adopt the same transformation as the heck crate in the future. Prior to this, I would like to release a version that will result in a compile error in the event of any incompatibility.

Steps

• Implement a feature that triggers a compile error when incompatibility arises.
• Release a version that triggers a compile error when incompatibility arises.
• Implement a transformation that is compatible with heck.
• Release a version that performs a transformation compatible with heck.