Encode a string to be emitted to an XDC file with specifiers derived from ReferenceTargets (RTs). RTs are called out in the format string using "{}" a la Python. This makes the emitted XDC more robust against the module hierarchy manipulations (such as promotion/extraction and linking) that Golden Gate performs.

For example: XDCAnnotation("get_pins -of [get_clocks {}]", clockRT)

Would eventually emit: get_pins -of [get_clocks absolute/path/to/clock]

Here the emission pass by default creates a full instance path to the reference to avoid multiple matches.

Restrictions: 1. Multiple specifiers in a string are permitted. However, to simplify emission under more complex duplication conditions, all RTs must share the same explicit root module (i.e _{their module parameter is the same). . 2. Specifiers currently cannot point at aggregates. This will be resolved in a future PR.}