Before a component can be instantiated (i.e. "plugged in"), it must be
declared either in a package, a library, or the architecture
declaration region. An example of the syntax is shown on this slide
for the component and_gate which requires two inputs,
in1 and in2, and one output, out1. Note that no
information about how this gate works is given. Generics can also be
included in the component declaration. The component declaration is
used to select the component that will be used in subsequent component
instantiations that appear in the architecture description.
