Preserve leading underscores in generated Idents

[eggyal]

heck documents, under Definition of a word boundary:

Characters not within words (such as spaces, punctuations, and underscores) are not included in the output string except as they are a part of the case being converted to. Multiple adjacent word boundaries (such as a series of underscores) are folded into one. (“hello__world” in snake case is therefore “hello_world”, not the exact same string). Leading or trailing word boundary indicators are dropped, except insofar as CamelCase capitalizes the first word.

As a consequence, where heck is used to construct a Rust Ident that might only be valid due to its leading underscore (such as here), the resulting Ident may be invalid. See rust-lang/rust#98158 for a report of an arising ICE.

strum should ensure that, in such cases, leading underscores are preserved (or else document that such identifiers are unsupported).