Implement proof of concept for power assert

PowerAssert.md

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+# Proof of Concept for Power Assert for Swift
+
+Power assertions (a.k.a. diagrammed assertions) augment your assertion failures with information about values produced during the evaluation of a condition, and presents them in an easily digestible form.
+Power assertions are a popular feature of [Spock](https://github.com/spockframework/spock) (and later the whole [Groovy](https://github.com/apache/groovy) language independently of Spock),
+[ScalaTest](http://www.scalatest.org/), and [Expecty](https://github.com/pniederw/expecty).
+
+Power assertions provide descriptive assertion messages for your tests, like this.
+
+```
+#powerAssert(max(a, b) == c)
+             |   |  |  |  |
+             7   4  7  |  12
+                       false
+
+#powerAssert(xs.contains(4))
+             |  |        |
+             |  false    4
+             [1, 2, 3]
+
+#powerAssert("hello".hasPrefix("h") && "goodbye".hasSuffix("y"))
+             |       |         |    |  |         |         |
+             "hello" true      "h"  |  "goodbye" false     "y"
+                                    false
+```
+
+
+Getting Started
+---------------------------------------
+
+#### Run Example
+
+You can run the example with the following command:
+
+```shell
+swift build && .build/debug/swift-parser-cli expand-macros PowerAssertTests.swift | swiftc - ./PowerAssert.swift && ./main && rm ./main
+```