feat(neon-macros): Convert snake_case to camelCase when exporting functions

[kjvalencik]

Rust uses a snake_case convention for functions while JavaScript camelCase. This changes makes the default behavior of neon::export convert to camelCase to follow JavaScript naming conventions.

If the user does not want this behavior the name = ".." override may be used to explicitly provide a name.

No mapping is provided for const or static exported globals since both Rust and JS follow a YELLING_SNAKE_CASE convention.

[dherman]

I like this, I think it's a better default. For the implementation, I'm proposing that we use convert_case. See the comments inline for my rationale.

[dherman]

We discussed this on Slack and came to the following conclusions:

• convert_case doesn't handle things like leading and trailing underscores
• Using an external crate is a bit risky since this is part of the public behavior of Neon
• Even using regex would be a bit complex so it ends up being fine to implement this by hand
• We want to be pretty conservative about what conventions we recognize; if they have nonstandard names we will just bail and not transform the name

In the end this means:

• Preserve leading and trailing underscores
• Only capitalize the first character after a non-leading underscore
• Bail if we see two underscores in a row in between words or a capital letter

[dherman]

This looks great. Ship it!