fix(Types): Move distinct types into distinct identifiers

This allows types to be collected; prior to this commit they
   were conflicting from different modules.

   Uses this fix to extend sqrt to bigint, with the convention
   that it is undefined for non-perfect squares when 'predictable'
   is false and is the "best" approximation to the square root when
   'predictable' is true. Furthermore, for negative bigints, you might
   get a Gaussian integer when predictable is false; or you will just get
   your argument back when 'predictable' is true because what other
   bigint could you give back for a negative bigint?

   Also had to modify tests on the sign in sqrt(Complex) and add functions
   'zero' and 'one' to get types to match, as expected in #27.

   Adds numerous tests.

   Resolves #26.
   Resolves #27.

test/_pocomath.mjs

@@ -22,11 +22,9 @@ describe('The default full pocomath instance "math"', () => {
          add: {
             '...stringK': () => addends => addends.reduce((x,y) => x+y, '')
          },
-         Types: {
-            stringK: {
-               test: s => typeof s === 'string' && s.charAt(0) === 'K',
-               before: ['string']
-            }
+         Type_stringK: {
+            test: s => typeof s === 'string' && s.charAt(0) === 'K',
+            before: ['string']
          }
       })
       assert.strictEqual(math.add('Kilroy','K is here'), 'KilroyK is here')