add Equal angle constraint + more cases for existing constraints (#69)

* add more setLength and horz vert distance constraints

* rename equal to equalLength

* Add equalAngle constraint

src/lang/std/std.ts

@@ -16,6 +16,7 @@ import {
 } from './sketch'
 import {
   segLen,
+  segAng,
   angleToMatchLengthX,
   angleToMatchLengthY,
   segEndX,
@@ -143,6 +144,7 @@ export const internalFns: { [key in InternalFnNames]: InternalFn } = {
   lastSegX,
   lastSegY,
   segLen,
+  segAng,
   angleToMatchLengthX,
   angleToMatchLengthY,
   lineTo: lineTo.fn,