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/generic/arithmetic.mjs

@@ -4,6 +4,7 @@ export * from './Types/generic.mjs'
 
 export const add = reducingOperation
 export {lcm} from './lcm.mjs'
+export {mean} from './mean.mjs'
 export {mod} from './mod.mjs'
 export const multiply = reducingOperation
 export {divide} from './divide.mjs'

src/generic/mean.mjs

@@ -0,0 +1,3 @@
+export const mean = {
+   '...any': ({add, divide}) => args => divide(add(...args), args.length)
+}

src/generic/multiply.mjs

@@ -1,13 +0,0 @@
-export * from './Types/generic.mjs'
-
-export const multiply = {
-   'undefined': () => u => u,
-   'undefined,...any': () => (u, rest) => u,
-   any: () => x => x,
-   'any,undefined': () => (x, u) => u,
-   'any,any,...any': ({self}) => (a,b,rest) => {
-      const later = [b, ...rest]
-      return later.reduce((x,y) => self(x,y), a)
-   }
-}
-