Adds methods for binding graph nodes and llvm values

Each node of the control graph may be associated with
llvm::Value object.

Method compiler iterates through the control graph
and creates values which then bound to graph nodes.

Some graph nodes refer other nodes as their arguments.
This relation is reflected by similar Value relations.
When Value creation require argument which had not
been processed yet, dummy value is inserted. This dummy
value will be replaced later when graph node holding it
will be processed.

Issue: #32