feat(Chain): add computation pipelines like mathjs

Also adds a `mean()` operation so there will be at least one operation
  that takes only a rest parameter, to exercise the ban on splitting
  such a parameter between the stored value and new arguments.
  Adds various tests of chains.

  Resolves #32.

src/core/Chain.mjs

@@ -0,0 +1,70 @@
+/* An object that holds a value and a reference to a PocomathInstance for
+ * applying operations to that value. Since the operations need to be wrapped,
+ * that instance is supposed to provide a place where wrapped operations can
+ * be stored, known as the repository.
+ */
+class Chain {
+   constructor(value, instance, repository) {
+      this.value = value
+      this.instance = instance
+      this.repository = repository
+      if (!('_wrapped' in this.repository)) {
+         this.repository._wrapped = {}
+      }
+   }
+
+   /* This is the wrapper for func which calls it with the chain's
+    * current value inserted as the first argument.
+    */
+   _chainify(func, typed) {
+      return function () {
+         // Here `this` is the proxied Chain instance
+         if (arguments.length === 0) {
+            this.value = func(this.value)
+         } else {
+            const args = [this.value, ...arguments]
+            if (typed.isTypedFunction(func)) {
+               const sigObject = typed.resolve(func, args)
+               if (sigObject.params.length === 1) {
+                  throw new Error(
+                     `chain function ${func.name} attempting to split a rest`
+                        + 'parameter between chain value and other arguments')
+               }
+               this.value = sigObject.implementation.apply(func, args)
+            } else {
+               this.value = func.apply(func, args)
+            }
+         }
+         return this
+      }
+   }
+}
+
+export function makeChain(value, instance, repository) {
+   const chainObj = new Chain(value, instance, repository)
+   /* Rather than using the chainObj directly, we Proxy it to
+    * ensure we only get back wrapped, current methods of the instance.
+    */
+   return new Proxy(chainObj, {
+      get: (target, property) => {
+         if (property === 'value') return target.value
+         if (!(property in target.instance)) {
+            throw new SyntaxError(`Unknown operation ${property}`)
+         }
+         if (property.charAt(0) === '_') {
+            throw new SyntaxError(`No access to private ${property}`)
+         }
+         const curval = target.instance[property]
+         if (typeof curval !== 'function') {
+            throw new SyntaxError(
+               `Property ${property} does not designate an operation`)
+         }
+         if (curval != target.repository._wrapped[property]) {
+            target.repository._wrapped[property] = curval
+            target.repository[property] = target._chainify(
+               curval, target.instance._typed)
+         }
+         return target.repository[property]
+      }
+   })
+}

src/core/PocomathInstance.mjs

@@ -1,6 +1,7 @@
 /* Core of pocomath: create an instance */
 import typed from 'typed-function'
 import dependencyExtractor from './dependencyExtractor.mjs'
+import {makeChain} from './Chain.mjs'
 import {subsetOfKeys, typesOfSignature} from './utils.mjs'
 
 const anySpec = {} // fixed dummy specification of 'any' type
@@ -21,6 +22,7 @@ export default class PocomathInstance {
     * must be added to this list.
     */
    static reserved = new Set([
+      'chain',
       'config',
       'importDependencies',
       'install',
@@ -64,6 +66,7 @@ export default class PocomathInstance {
             return true // successful
          }
       })
+      this._chainRepository = {} // place to store chainified functions
    }
 
    /**
@@ -164,6 +167,11 @@ export default class PocomathInstance {
       return result
    }
 
+   /* Return a chain object for this instance with a given value: */
+   chain(value) {
+      return makeChain(value, this, this._chainRepository)
+   }
+
    _installInstance(other) {
       for (const [type, spec] of Object.entries(other.Types)) {
          if (type === 'any' || this._templateParam(type)) continue