refactor: Simpler merging mechanism

Merging of Pocomath modules is eased by allowing one PocomathInstance to
  be merged into another. That allows types, for example, to be exported
  as a PocomathInstance (so there is no need for a special identifier
  convention for types; they can be directly added with an installType
  method). Also, larger modules can just be exported as an instance, since
  there is more flexibility and more checking in merging PocomathInstances
  than raw modules.

src/complex/Types/Complex.mjs

@@ -1,3 +1,5 @@
+import PocomathInstance from '../../core/PocomathInstance.mjs'
+
 /* Use a plain object with keys re and im for a complex; note the components
  * can be any type (for this proof-of-concept; in reality we'd want to
  * insist on some numeric or scalar supertype).
@@ -6,11 +8,13 @@ function isComplex(z) {
    return z && typeof z === 'object' && 're' in z && 'im' in z
 }
 
-export const Type_Complex = {
+const Complex = new PocomathInstance('Complex')
+Complex.installType('Complex', {
    test: isComplex,
    from: {
       number: x => ({re: x, im: 0}),
       bigint: x => ({re: x, im: 0n})
    }
-}
+})
 
+export {Complex}

src/complex/all.mjs

@@ -1,5 +1,5 @@
-export * from '../generic/arithmetic.mjs'
-export * from './native.mjs'
+import PocomathInstance from '../core/PocomathInstance.mjs'
+import * as complexes from './native.mjs'
+import * as generic from '../generic/arithmetic.mjs'
 
-// resolve the conflicts
-export {sqrt} from './sqrt.mjs'
+export default PocomathInstance.merge('complex', complexes, generic)

src/complex/sqrt.mjs

@@ -35,7 +35,6 @@ export const sqrt = {
          const reSign = sign(z.re)
          if (imSign === imZero && reSign === reOne) return self(z.re)
          const reTwo = add(reOne, reOne)
-         const partial = add(abs(z), z.re)
          return complex(
             multiply(sign(z.im), self(divide(add(abs(z),z.re), reTwo))),
             self(divide(subtract(abs(z),z.re), reTwo))