Add Definition and Instance API (#2045)

This introduces a new experimental API for module instantiation that disentagles
elaborating the definition (or implementation) from instantiation of
a given module. This solves Chisel's longstanding reliance on
"Deduplication" for generating Verilog with multiple instances of the
same module.

The new API resides in package chisel3.experimental.hierarchy. Please
see the hierarchy ScalaDoc, documentation, and tests for examples of
use.

Co-authored-by: Jack Koenig <koenig@sifive.com>
Co-authored-by: Megan Wachs <megan@sifive.com>
Co-authored-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>