Add Pointer.pointer

This method is used to create a pointer to a pointer, relying on Inko's
memory layout to make this possible. Pointer pointers are needed for C
APIs that rely on output pointers, such as the SQLite API.

Changelog: added

libstd/src/std/ffi.inko

@@ -22,6 +22,9 @@ import std::set::Set
 # The address used for NULL pointers.
 let NULL_ADDRESS = 0x0
 
+# The size (in bytes) of an Inko object header.
+let HEADER_SIZE = 16
+
 # A pointer to a C value.
 class pub Pointer {
   # The raw pointer.
@@ -75,6 +78,14 @@ class pub Pointer {
   fn pub raw -> Any {
     @raw
   }
+
+  # Returns a pointer to the raw pointer wrapped by `self` (aka a pointer
+  # pointer).
+  fn pub pointer -> Pointer {
+    let addr = _INKO.ffi_pointer_address(self) + HEADER_SIZE
+
+    Pointer.from_address(addr)
+  }
 }
 
 impl Clone for Pointer {

libstd/test/std/test_ffi.inko

@@ -81,6 +81,15 @@ fn pub tests(t: mut Tests) {
     t.equal(mem.ptr.read(type: Type.U8, offset: 0) as Int, 10)
   }
 
+  t.test('Pointer.pointer') fn (t) {
+    let a = Pointer.null
+    let b = a.pointer
+
+    b.write(type: Type.U8, offset: 0, value: 10)
+    t.equal(a.address, 10)
+    t.not_equal(b.address, 10)
+  }
+
   t.test('Pointer.clone') fn (t) {
     let ptr = Pointer.from_address(0x4)