Improve performances

Following #444, I bring perf improvements from @smooth-ui/system into styled-system.
The goal is to make this library the best one and the universal one.

What have changed?

- I switched to the core of @smooth-ui/system, more performant
especially with multiple compose. Composition is now flat, it means you
can compose 10 times and it will not be slower. We are probably in O(n),
where "n" is the number of props.
- I splitted the logic in several files to make it clearer
- I switched to rollup in order to optimize build, provide umd and
improve tree shaking

What are the breaking changes?

- We now rely on the order of props. ES2015 is here and we can rely on
it, see this tweet for context: https://twitter.com/ianstormtaylor/status/1115680037875752963.
- We are more strict in breakpoints, we don't accept a non-existing
breakpoint
- We don't export utilities like get, themeGet, etc...
- PropTypes are no longer included, they can be generated from meta
- We don't return array, just an object with all props merged
- Properties that default to pixels unit (like margin, padding) no
longer include it

.npmignore

@@ -1,14 +1,3 @@
-.DS_Store
-.prettierrc
-.nyc_output
-.travis.yml
-coverage
-coverage.lcov
-bench
-docs
-src
-examples
-babel.config.js
-test
-CONTRIBUTING.md
-CODE_OF_CONDUCT.md
+/*
+!/dist/**/*.js
+!/props.js

babel.config.js

@@ -1,29 +1,16 @@
-module.exports = {
-  env: {
-    test: {
-      presets: [
-        [ '@babel/env', { loose: true } ],
-        '@babel/react'
-      ],
-      plugins: [
-        '@babel/transform-runtime'
-      ]
-    },
-    cjs: {
-      presets: [
-        [ '@babel/env', { loose: true } ]
-      ],
-      plugins: [
-        '@babel/transform-runtime'
-      ]
-    },
-    esm: {
-      presets: [
-        [ '@babel/env', { loose: true, modules: false } ]
-      ],
-      plugins: [
-        ['@babel/transform-runtime', { useESModules: true }]
-      ]
-    },
+module.exports = function getConfig(api) {
+  const isRollup = api.caller(
+    caller => caller && caller.name === 'rollup-plugin-babel'
+  )
+  if (!isRollup)
+    return {
+      presets: [['@babel/preset-env', { loose: true }], '@babel/react'],
+    }
+  return {
+    presets: [['@babel/preset-env', { loose: true, modules: false }]],
+    plugins: [
+      'babel-plugin-annotate-pure-calls',
+      ['@babel/transform-runtime', { useESModules: true }],
+    ],
   }
 }

benchmarks/index.js

@@ -0,0 +1,40 @@
+/* eslint-disable no-console, import/no-unresolved */
+const Benchmark = require('benchmark')
+
+const libs = [
+  {
+    name: 'actual',
+    module: require('../dist/styled-system.cjs'),
+  },
+  {
+    name: 'v4',
+    module: require('./v4.1.0'),
+  },
+].map(({ name, module: { compose, fontSize, space } }) => {
+  const system = compose(
+    fontSize,
+    space
+  )
+  const run = () => system({ p: [10, 20], mt: 10, m: '20px', fontSize: 10 })
+  return { name, run }
+})
+
+const suite = new Benchmark.Suite()
+
+console.log('Initial run...')
+
+libs.forEach(lib => {
+  console.log(lib.name, lib.run())
+  suite.add(lib.name, lib.run)
+})
+
+console.log('Run suite...')
+
+suite
+  .on('cycle', event => {
+    console.log(String(event.target))
+  })
+  .on('complete', function onComplete() {
+    console.log(`Fastest is ${this.filter('fastest').map('name')}`)
+  })
+  .run({ async: true })