feat: Generic numerical types

Inspired by https://github.com/josdejong/mathjs/discussions/2212 and
  https://github.com/josdejong/mathjs/issues/2585.

  Provides a simple adapter function `adapted` which takes a class
  implementing an arithmetical datatype and returns a PocomathInstance
  with a new type for that class, invoking the methods of the class
  in a standard way for the Pocomath/mathjs operations.  (That instance
  can then be installed in another to add the new type to any instance
  you like, including the default one.)

  Uses this facility to bring fraction.js Fraction into Pocomath, and
  tests the resulting type.

  Currently the "standard" interface for an arithmetical type is heavily modeled
  after the design of fraction.js, but with experience with other 3rd-party types
  it could be streamlined to something pretty generic (and the Fraction
  adaptation could be patched to conform to the resulting "standard"). Or a
  proposal along the lines of https://github.com/josdejong/mathjs/discussions/2212
  could be adopted, and a shim could be added to fraction.js to conform to
  **that** standard.

  Resolves #30.

src/core/PocomathInstance.mjs

@@ -263,6 +263,7 @@ export default class PocomathInstance {
     * @param {{test: any => bool,  // the predicate for the type
     *          from: Record<string, <that type> => <type name>> // conversions
     *          before: string[]  // lower priority types
+    *          refines: string   // The type this is a subtype of
     *        }} specification
     *
     * The second parameter of this function specifies the structure of the
@@ -711,7 +712,12 @@ export default class PocomathInstance {
                   if (argType === 'any') {
                      throw TypeError(
                         `In call to ${name}, incompatible template arguments: `
-                           + args.map(a => JSON.stringify(a)).join(', '))
+                        // + args.map(a => JSON.stringify(a)).join(', ')
+                        // unfortunately barfs on bigints. Need a better formatter
+                        // wish we could just use the one that console.log uses;
+                        // is that accessible somehow?
+                           + args.map(a => a.toString()).join(', ')
+                           + ' of types ' + argTypes.join(', ') + argType)
                   }
                   argTypes.push(argType)
                }
@@ -947,7 +953,7 @@ export default class PocomathInstance {
             } else {
                throw new Error(
                   'Implement inexact self-reference in typed-function for '
-                     + neededSig)
+                     + `${name}(${neededSig})`)
             }
          }
          const refs = imps[aSignature].builtRefs