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 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.
- Object keys or array keys have now equivalent parsing

.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

.size-snapshot.json

@@ -0,0 +1,21 @@
+{
+  "dist/styled-system.cjs.js": {
+    "bundled": 18794,
+    "minified": 11425,
+    "gzipped": 3172
+  },
+  "dist/styled-system.esm.js": {
+    "bundled": 16608,
+    "minified": 9477,
+    "gzipped": 2776,
+    "treeshaked": {
+      "rollup": {
+        "code": 79,
+        "import_statements": 44
+      },
+      "webpack": {
+        "code": 1112
+      }
+    }
+  }
+}

babel.config.js

@@ -1,29 +1,7 @@
 module.exports = {
   env: {
     test: {
-      presets: [
-        [ '@babel/env', { loose: true } ],
-        '@babel/react'
-      ],
-      plugins: [
-        '@babel/transform-runtime'
-      ]
+      presets: [['@babel/preset-env', { loose: true }], '@babel/react'],
     },
-    cjs: {
-      presets: [
-        [ '@babel/env', { loose: true } ]
-      ],
-      plugins: [
-        '@babel/transform-runtime'
-      ]
-    },
-    esm: {
-      presets: [
-        [ '@babel/env', { loose: true, modules: false } ]
-      ],
-      plugins: [
-        ['@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 })