Add an implementation written in Vala.

Add an implementation written in Vala.

I needed to learn Vala in order to work on an existing program written
in it, and since Ben Harris had been enthusing about mal to me
recently, I thought this would be a good way to learn the language.

In other words, this is my first Vala project, so I can't promise it's
perfectly idiomatic by the standards of experienced Vala programmers!

Also, the style isn't all that consistent. For example, sometimes I
implement a function for every value type by adding a method to the
Mal.Val base class, and sometimes instead I do it by having a function
(e.g. pr_str) dynamically test all the possible subtypes in a chain of
else-ifs.

Those decisions were partly because of the source file structure
required by this project (pr_str is supposed to live in its own file);
partly because for my own learning goals I wanted to try both
strategies and find out how easy they both were; and mostly just
because of indecision and bad planning.