feat: Start adding vector arithmetic

So far, abs, add, norm, normsq, and sum are supported. To get them
  to work, also implements the following:
  * refactor: Use ReturnType function rather than just accessing .returns
  * feat: distinguish marking a function as a behavior from its return type
  * refactor: Rename `NotAType` to `Unknown` because it must be made closer
    to a bona fide type for the sake of inhomogeneous vectors
  * feat: make resolving a TypeDispatcher method on a type vector including
    `Unknown` into a no-op; that simplifies a number of generic behaviors
  * feat: add `haszero` method parallel to `hasnan`
  * feat: track the Vector nesting depth of Vector specializations

src/generic/__test__/arithmetic.spec.js

@@ -1,6 +1,6 @@
 import assert from 'assert'
 import math from '#nanomath'
-import {ReturnTyping} from '#core/Type.js'
+import {ReturnType, ReturnTyping} from '#core/Type.js'
 
 const {Complex, NumberT} = math.types
 
@@ -8,7 +8,7 @@ describe('generic arithmetic', () => {
    it('squares anything', () => {
       const sq = math.square
       assert.strictEqual(sq(7), 49)
-      assert.strictEqual(math.square.resolve([NumberT]).returns, NumberT)
+      assert.strictEqual(ReturnType(math.square.resolve([NumberT])), NumberT)
       assert.deepStrictEqual(sq(math.complex(3, 4)), math.complex(-7, 24))
       const eyes = math.complex(0, 2)
       assert.strictEqual(sq(eyes), -4)