Generate bindings for private extern functions by default

[mversic]

Rust compiler will always export private extern functions. We can say that visibility rules don't apply for extern functions. Considering this, generating C bindings for private extern functions should be opt-out. In the meantime, is there some way to opt-in generating bindings for private functions?

[mversic]

to make a more convincing case I'm providing steps to reproduce:

1. I made a project as follows:

[package]
name = "example"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ['cdylib']

[profile.release]
lto = true           # NOTE: I've even enabled LTO 

#[no_mangle]        // NOTE: If `no_mangle` is removed the function will not be exported
extern fn i_am_exported() {}

2. I built the library with cargo build --release which produced target/release/libexample.dylib
3. I inspected the produced library with objdump -t target/release/libexample.dylib which shows:

target/release/libexample.dylib:	file format mach-o arm64

SYMBOL TABLE:
0000000000000000      d  *UND* 
0000000000000000      d  *UND* /rustc/d5a82bbd26e1ad8b7401f6a718a9c57c96905483/library/alloc/src/lib.rs/@/
0000000000000000      d  *UND* alloc.515a7714-cgu.0
0000000000000000      d  *UND* /Users/mversic/soramitsu/example/target/release/deps/example.example.ae59a36c-cgu.0.rcgu.o
0000000000003fb4      d  *UND* 
0000000000003fb4      d  *UND* _i_am_exported
0000000000000004      d  *UND* 
0000000000003fb4      d  *UND* 
0000000000000000      d  *UND* 
0000000000003fb4 g     F __TEXT,__text _i_am_exported

I hope this makes a convincing enough case that current default behavior of cbindgen is not aligned with rusts default behavior

NOTES:

1. you can see in the symbol table that _i_am_exported is gF -> a global function
2. It is interesting to note that no_mangle affects the final result. If this attribute is not present, the function will never be exported regardless if it's public or private.

[mversic]

merged