Adds missing flag values to reflection API

Among the flags requested by our users, the most popular are definitely
PARAMACCESSOR and CASEACCESSOR. Among other things that are missing in
2.10.x are SYNTHETIC and its close friend ARTIFACT. Apparently, SYNTHETIC
was was added somewhen during the 2.11.0-M* development cycle, so I only
has to introduce ARTIFACT.

There was also METHOD as requested by Paul in https://github.com/xeno-by/declosurify/blob/ce6c55b04086c755102227dd8124b0bf0dac46e1/src/main/scala/macros.scala#L102,
but the absence of that one is compensated by the fact that it’s only needed
when manually creating symbols, and for that we already have Symbol.newMethodSymbol.