feat: Allow self-reference in implementations

And use it to define negate and add for Complex numbers in a way that
  is independent of component types.

  Also add a bigint type and verify that pocomath will then handle Gaussian
  integers "for free".

bigint/BigInt.mjs

@@ -0,0 +1,3 @@
+import typed from 'typed-function'
+
+typed.addType({name: 'bigint', test: b => typeof b === 'bigint'})

bigint/add.mjs

@@ -0,0 +1,4 @@
+import './BigInt.mjs'
+export const add = {
+   '...bigint': [[], addends => addends.reduce((x,y) => x+y, 0n)],
+}

bigint/all.mjs

@@ -0,0 +1,3 @@
+export {add} from './add.mjs'
+export {negate} from './negate.mjs'
+export {subtract} from '../generic/subtract.mjs'

bigint/negate.mjs

@@ -0,0 +1,2 @@
+import './BigInt.mjs'
+export const negate = {bigint: [[], b => -b ]}