Inlining uses "_", respects namespaces

Summary of changes:
  - Use "_" as an inlining delimiter instead of "$"
  - Makes inlining avoid namespace conflicts

This changes the delimiter used for inlining to "_" instead of "$". This
avoids problems with buggy parsers that may not handle "$" correctly. As
ClockListTransform relies on the explicit use of "$", the delimiter is a
FIRRTL-private val that the ClockListTransform overrides (to the original
"$").

Namespace conflicts could occur previously, but are very rare as users
will almost never use "$" in a name (even though it's allowed by both the
FIRRTL and Verilog specifications). Moving to "_" increases the
probability of namespace conflicts occurring. This adds explicit checking
that inlined names will not introduce namespace conflicts and that
generated names are prefix unique (as defined in the spec).

Note: inlined modules may not have unique prefixes. A test is included
that this is the case and an ignored test shows what prefix uniqueness
would look...