Add the loader package.
Add a simple tests for loader.
Associate an ast.Node with each ssa.Value and ssa.Instruction.
Simplify the loader.Program type's API.
Add loader.Program.Comments.

It maps ast.Nodes to their associated comment groups.
Add a test for Program.Node and Program.Comments.
Some small comment fixes.
Merge pull request #1 from eaburns/loader

Add the loader package.