- event expressions use Expr over LHS
- stricter AST representation of event controls
- property specs use event expressions directly

- scoped traversals can now raise errors which contain the path of the
  current scope and an approximate source location based on preceding
  trace comments, if available
- initially, this new error messaging has only been added for the
  illegal size cast checks in the TypeOf and Cast conversions
- error suite tests can provide a verbose mode expected source location

- error suite runs in both regular and verbose mode
- add helper for extracting "flags" from test files
- common assertMatch, assertNotMatch helpers
- remove unused assertExists helper
- fix un-escaped wildcard in assert_deferred_nonzero.sv