Add serde functionality into lib. (#422)

* Adds decoration to the verifier errors.

example:

function %bad(i32) fast {
ebb0(v0: i32):
    brnz.i32 v0, ebb1
    return
    ^~~~~~

verifier inst1: Internal return not allowed with return_at_end=1

ebb1:
    trapz.i32 v0, user6
    return
}

Fixes #68

* Making it so that pretty_function_error and write_function_plain are both private.

* Changes write_ebb to decorate_ebb.
Adds documentation line to decorate_function.

* Removing Cargo.toml lib/serde addition

* Add serde functionality into lib.

* Fix so code is compatible with Rust version 1.25.0.

* Move ser/de functions to utility file, update description, remove borrow from arms.

* Remove commented out code.

lib/serde/README.md

@@ -0,0 +1,30 @@
+This crate performs serialization of the [Cranelift](https://crates.io/crates/cranelift) IR.
+
+This crate is structured as an optional ability to serialize and deserialize cranelift IR into JSON format.
+
+Status
+------
+
+Cranelift IR can be serialized into JSON.
+
+Deserialize is a work in progress, as it currently deserializes into the serializable data structure that can be utilized by serde instead of the actual Cranelift IR data structure. 
+
+
+Building and Using Cranelift Serde
+----------------------------------
+
+clif-json usage: 
+
+    clif-json serialize [-p] <file>
+    clif-json deserialize <file>
+
+Where the -p flag outputs Cranelift IR as pretty JSON.
+
+For example to build and use clif-json:
+
+``` {.sourceCode .sh}
+cd lib/serde
+cargo build
+clif-json serialize -p test.clif
+```
+