fix Complex type check
modify Matrix.toString()
Interpreter: disable debug output
slightly better error message on typing
add classes to describe provided functions and procs
add CodeProvider interface (and factory)
add EmptyProvider implementation of CodeProvider
use CodeProvider in MatrixGlobalScope
adapt interpretation to CodeProviders
add AbstractDeclarativeProvider implementation

Similar to many XText classes, this allows adding
new provided procs and functions by extending the
class and adding specifically named methods that
are retrieved via reflection and later called via
polymorphic dispatch.
add BuiltinCodeProvider with builtin print procs

Fixes #29.