Add a derive_constructor option to generate struct constructors

tests/expectations/annotation.cpp

@@ -9,6 +9,10 @@ enum class C : uint32_t {
 struct A {
   int32_t m0;
 
+  A(int32_t const& aM0)
+    : m0(aM0)
+  {}
+
   bool operator<(const A& other) const {
     return m0 < other.m0;
   }

tests/rust/annotation.rs

@@ -1,5 +1,6 @@
 /// cbindgen:derive-lt=true
 /// cbindgen:derive-lte=true
+/// cbindgen:derive-constructor=true
 /// cbindgen:rename-all=GeckoCase
 #[repr(C)]
 struct A(i32);