Function serialize_request 

pub fn serialize_request(
    method: &str,
    path: &str,
    headers: &[(String, String)],
    body: &[u8],
) -> Vec<u8> 

Serialize a request into the nano-service wire format (all lengths u32 big-endian):

u32 method_len, method
u32 path_len, path
u32 headers_count, [u32 key_len, key, u32 val_len, val]...
u32 body_len, body