Make crate used in entry_point configurable

[aumetra]

Replaces #1539

This PR makes the crate used in the expansion of the entry_point macro configurable.
But with actual error handling and a nice syntax.

Rename example:

#[entry_point(crate = "::my_crate::cw_std_reexport")]

It also rejects other attributes and syntaxes:

#[entry_point(other = "attribute")] // This is rejected
#[entry_point(::my_crate::cw_std_reexport)] // This is also rejected

TODO:

• Tests

[aumetra]

Whether the attribute is a string or a path can be changed pretty easily, so whatever fits better into the desired look

[uint]

Small change, but very nicely done. LGTM, thanks!

[uint]

Whether the attribute is a string or a path

AFAIK what you've done is idiomatic, so all good!

[webmaster128]

#[entry_point(crate = "::my_crate::cw_std_reexport")]

Great stuff. But one naming thing: Aren't we setting the "module" there, not the "crate"?

[aumetra]

But one naming thing: Aren't we setting the "module" there, not the "crate"?

Technically yes, you can set a module here and not only a crate. But I took the naming in this case from serde (https://serde.rs/container-attrs.html#crate) since it is probably most familiar to users

[webmaster128]

okay, sure. Thanks for the clarification. I don't expect sub-module usages here anyways. From the very few users of the option, the majority puts in fact a different crate name in it.