add experimental refraction module (#10)

Squashed:

* add experimental refraction module
* review round 1
* do not require caller to import std.traits
* more doc and unit test tweaks
* review round 2
* review round 3
* review round 4
* in code, use __MODULE__ instead of spelling it out
* avoid breaking the 80th column wall as much as possible

README.md

@@ -21,6 +21,7 @@ Bolts is a utility library for the D programming language which contains a numbe
 * **iz**: super non-eponymous template that provides a lot of the functionality that's in the traits module with a different sytax that allows their usage in meta functions as well.
 * **experimental**: contains experimental features
     *signatures: working implementation of type signatures
+    *refraction: generate mixin strings to replicate a function, with some changes
 
 Most functions here operate on any compile time entity. For example `isUnaryOver` works in both these situatons:
 
@@ -82,3 +83,26 @@ source/bolts/experimental/signatures.d(310,5): Error: static assert:  "Type MyRa
   source/bolts/experimental/signatures.d(464): <-- Signature InputRange!(int) defined here.
   source/bolts/experimental/signatures.d(471): <-- Checked here."
 ```
+
+## Refraction (experimental):
+
+It is sometimes necessary to create a function which is an exact copy of
+another function. Or sometimes it is necessary to introduce a few variations,
+while carrying all the other aspects. Because of function attributes, parameter
+storage classes and user-defined attributes, this requires building a string
+mixin. In addition, the mixed-in code must refer only to local names, if it is
+to work across module boundaires. This module facilitates the creation of such
+mixins.
+
+For example, this creates a function that has a different name and return type,
+but retains the 'pure' attribute from the original function:
+
+```d
+pure int answer() { return 42; }
+mixin(
+  refract!(answer, "answer").withName("realAnswer")
+  .withReturnType("real")
+  .mixture);
+static assert(is(typeof(realAnswer()) == real));
+static assert(functionAttributes!realAnswer & FunctionAttribute.pure_);
+```

dub.json

@@ -9,5 +9,14 @@
     "targetPath": "bin",
     "toolChainRequirements": {
         "frontend": ">=2.086"
-    }
+    },
+	"configurations": [
+		{
+			"dflags": [
+				"-mixin=mixins.txt"
+			],
+			"name": "unittest",
+			"targetType": "library"
+		}
+	],
 }