Add an environment struct and associated functions
Filling in the eval function...
An initial (incomplete) parser and pretty-printer.
Convert lambda syntax to use fn, and remove {Quote,Lambda}Expr in favour of the Value equivalents
Move the ToStr impl for Ident
Merge pull request #1 from huonw/parsing

An initial (incomplete) parser and pretty-printer.
Remove unused import
Remove stray pipe tests
Remove '-Expr' suffix from Expr variants
Update comment
Use refs in match patterns
Add do primitive
Use owned pointers
Add LICENSE and AUTHORS files
Rename value variants, remove environment field from lambda
Variant renaming (again!), Add a Rust fn Value variant.

Unfortunately deriving Eq and Clone seems to fail with the Rust variant. I had to implement them manually. Unfortunately this now means the tests fail to compile now :(
Make eval a method of Env as opposed to Expr
Rename Let to Def

'let' has a different meaning in lisp, so this was confusing
Make Symbol a varient of Value
Rename Call to Apply