Add utf_codepoint to stdlib

Author: tomwhatmore

src/gleam/utf_codepoint.gleam

@@ -0,0 +1,20 @@
+import gleam/result
+
+pub type UtfCodepoint = UtfCodepoint
+
+pub type Error {
+  Invalid
+}
+
+external fn int_to_utf8_codepoint(Int) -> UtfCodepoint =
+  "gleam_stdlib" "identity"
+
+pub fn from_int(value: Int) -> Result(UtfCodepoint, Error) {
+  case value {
+    i if i > 1114111 -> Error(Invalid)
+    i if i == 65534 -> Error(Invalid)
+    i if i == 65535 -> Error(Invalid)
+    i if i >= 55296 && i <= 57343 -> Error(Invalid)
+    i -> Ok(int_to_utf8_codepoint(i))
+  }
+}

test/utf_codepoint_test.gleam

@@ -0,0 +1,16 @@
+import gleam/should
+import gleam/utf_codepoint
+
+pub fn from_int_test() {
+  utf_codepoint.from_int(1114444)
+  |> should.be_error
+
+  utf_codepoint.from_int(65534)
+  |> should.be_error
+
+  utf_codepoint.from_int(55296)
+  |> should.be_error
+
+  assert Ok(snake) = utf_codepoint.from_int(128013)
+  should.equal(<<snake:utf8_codepoint>>, <<"🐍":utf8>>)
+}