Simplified FFI authoring with auto-generated C++ and Rust bindings

[Ivansete-status]

Summary

This replaces the manual, boilerplate FFI workflow with a pragma-based API and a compile-time binding generator. A library author now annotates regular Nim procs with {.ffi.} / {.ffiCtor.}, calls genBindings(), and gets complete C++ and Rust bindings.

closes #10

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Major changes

New compilation flags

• ffiGenBindings: when added, activates binding generation.
• targetLang (=cpp or =rust) selects the output desired language.
• ffiOutputDir=<path> : where to put the generated code.

Notice that, in order to make it work properly, the genBindings() must be invoked at the end of all the annotated {.ffi.} procs. For example, something like:

# mylib.nim  <-- the file you pass to `nim c`
import mylib/timer_api
import mylib/network_api
import mylib/storage_api

genBindings()

New macro API

Pragma	Purpose
{.ffi.} on a proc	Exports a method; async or sync path chosen automatically by detecting await
{.ffiCtor.} on a proc	Exports a constructor that creates and returns the FFI context
{.ffi.} on a type	Registers the type for binding generation
genBindings()	Emits C++ or Rust bindings at compile time (no-op without -d:ffiGenBindings)

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Sync path optimization

{.ffi.} pragma inspects the proc body at compile time. If no await is found then the macro generates a fast inline path that invokes the callback without enqueuing a request to the FFI thread channel.

Auto-generated C++ bindings (ffi/codegen/cpp.nim)

• Struct types with NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE macros
• extern C declarations
• Allows async C++ behaviour by the use of std:future.

Notice the genbindings_cpp task defined in examples/nim_timer/nim_timer.nimble, which allows auto-generating C++ bindings.

Auto-generated Rust bindings (ffi/codegen/rust.nim)

A complete Rust crate is emitted under the configured output directory.

Notice the genbindings_rust task defined in examples/nim_timer/nim_timer.nimble, which allows auto-generating Rust bindings.

Example

A self-contained example under examples/nim_timer/ demonstrating:

A constructor ({.ffiCtor.}) with a TimerConfig argument
An async method (nimtimerEcho) exercising chronos sleepAsync
A sync method (nimtimerVersion) taking the fast inline path
A method with complex nested types (ComplexRequest with seq, Option, type alias Maybe)
A nimtimer_destroy proc for clean teardown
A C++ client (cpp_bindings/main.cpp) and a Rust client (rust_client/) with both blocking and tokio async calls

[fcecin]

LGTM!