fix: Separate typed instance for each PocomathInstance (#15)

Also starts each PocomathInstance with no types at all, and uses the new
  situation to eliminate the need for a Complex "base case".

  Resolves #14.
  Resolves #13.

Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: #15

src/bigint/Types/bigint.mjs

@@ -0,0 +1,3 @@
+export const Types = {
+    bigint: {test: b => typeof b === 'bigint'}
+}

src/bigint/add.mjs

@@ -0,0 +1,5 @@
+export {Types} from './Types/bigint.mjs'
+
+export const add = {
+   '...bigint': [[], addends => addends.reduce((x,y) => x+y, 0n)],
+}

src/bigint/all.mjs

@@ -0,0 +1,4 @@
+export {Types} from './Types/bigint.mjs'
+export {add} from './add.mjs'
+export {negate} from './negate.mjs'
+export {subtract} from '../generic/subtract.mjs'

src/bigint/negate.mjs

@@ -0,0 +1,3 @@
+export {Types} from './Types/bigint.mjs'
+
+export const negate = {bigint: [[], b => -b ]}