jextract/ffm: Treat underscore labels as empty suffixes in function overloads

[sidepelican]

Currently, when a function overload uses an underscore (_) as an argument label, the generator uses the underscore literally as a suffix for the Java method name.

func foo(a: Int)    // in Java, void fooA(long a)
func foo(_ b: Int)  // in Java, void foo_(long b)

In Swift, _ is not an identifier; it simply denotes the omission of a label. Therefore, IMO, using it as a literal suffix in Java feels unnatural.

This PR updates the naming logic for function overloads to treat the _ label as an empty suffix.

Notes

This change can lead to name collisions in specific cases. For example:

public func takeValue(_ a: Int, b: Int) -> Int { 0 }
public func takeValue(b: Int, _ c: Int) -> Int { 0 }

With this PR, both methods would map to takeValueB.
However, even if we kept the underscore (the previous behavior), collisions would still be unavoidable if a function like func takeValue_(b: Int) existed.
To approach such corner-case collisions, we should eventually provide a way for users to explicitly specify the export name. (like @JavaExport("export_name")).

[ktoso]

This sounds good to me, long term solution as well, thank you!

Maybe worth a new issue about the longer term fix