refactor(rln): add V3 FFI module + migrate C/Nim examples

Author: vinhtc27

rln/ffi_c_examples/Readme.md

@@ -8,7 +8,7 @@ This example demonstrates how to use the RLN C FFI in stateful, stateless, and m
 
 ```bash
 cargo build -p rln --release
-cargo run --features headers --bin generate-headers
+cargo run --features headers --bin generate_headers
 mv -v rln.h rln/ffi_c_examples/
 ```
 
@@ -26,7 +26,7 @@ gcc -Wall main.c -o main -lrln -L../../target/release
 
 ```bash
 cargo build -p rln --release --no-default-features --features stateless
-cargo run --no-default-features --features stateless,headers --bin generate-headers
+cargo run --no-default-features --features stateless,headers --bin generate_headers
 mv -v rln.h rln/ffi_c_examples/
 ```
 
@@ -44,7 +44,7 @@ gcc -Wall -DSTATELESS main.c -o main -lrln -L../../target/release
 
 ```bash
 cargo build -p rln --release
-cargo run --features headers --bin generate-headers
+cargo run --features headers --bin generate_headers
 mv -v rln.h rln/ffi_c_examples/
 ```