Add private fullName member to Layer

Add a memoized (via a `lazy val`) member to Layer that returns the FIRRTL
"full name" of that layer.  This includes all the parent layers prefixed
with interleaves periods.

This is memoized to avoid unnecessarily recomputing the partial prefixes
of parents.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>