Rename items with a rule

[cormacrelf]

Everything else can have a rename rule. Why not items too!

Example below. Nice and neat, no #![allow(non_camel_case_types)] required.

#[repr(c)]
struct StructDef {
    x: i32,
    y: f32,
}
type TypeAlias = StructDef;
// doesn't apply to functions, obviously
// also this PR fixes misleading example in docs for [export.rename] which purported to apply to a function name
#[no_mangle]
extern "C" fn capi_write(obj: *mut StructDef) {}

[export]
prefix = "capi_"
rename_items = "snake_case"

typedef struct {
    int32_t x;
    float y;
} capi_struct_def;
typedef capi_struct_def capi_type_alias;
void capi_write(capi_struct_def *def);

[cormacrelf]

I realised this is a dupe of #419. This one has tests though.

[cormacrelf]

This is basically the use case for allowing one rule per item type as well as a default global one. I don't often see lower_snake_case #defines in C, even in a largely lower_snake_case API. This PR already has the infrastructure for that, passing IdentifierType in to ExportConfig::rename, but hasn't implemented it (yet).

[emilio]

So this looks probably ok, but wouldn't it be more consistent to just add a ConstantConfig::rename field of sorts? I think the point was that some constants might want to be static and can't be renamed, so that might be a bit trickier.

[orium]

Hi. Any change of getting this merged/release soon?