KCL stdlib and circle function (#1029)

Allows stdlib functions to be written as KCL, not as Rust. 

Rust stdlib functions will hereafter be referred to as "core" not "std".

Right now the only stdlib function I implemented is a circle function (it's a wrapper around the core arc function which sets the arc's start/end to 0 and 360 respectively). I know I want to change this function as soon as KCL has enums, which is my next task. So, I don't want users to start using this right away. To that end, I've named this function "unstable_stdlib_circle" not "circle". Once the function is ready to be stabilized, I can rename it to just "circle".

Note that this PR modifies the existing "sketch and extrude a cylinder" KCL test so that instead of using a user-defined circle function, it uses the unstable_stdlib_circle function now. And the twenty-twenty tests pass, so we know my stdlib is working.

https://github.com/KittyCAD/modeling-app/issues/922

src/wasm-lib/tests/executor/inputs/cylinder.kcl

@@ -1,14 +1,2 @@
-fn circle = (plane, center, radius) => {
-  const sg = startSketchOn(plane)
-    |> startProfileAt([center[0] + radius, center[1]], %)
-    |> arc({
-       angle_end: 360,
-       angle_start: 0,
-       radius: radius
-     }, %)
-    |> close(%)
-  return sg
-}
-
-const cylinder = circle('XY', [0,0], 22) |> extrude(14, %)
+const cylinder = unstable_stdlib_circle('XY', [0,0], 22) |> extrude(14, %)
 show(cylinder)