configuration benchmark iterations
Implicits: Move shadowing checkes after plausibility checks

In the name of optimization.
Don't use runtime reflection from the batch compiler.

Not only does this save a big chunk of time on startup by avoiding
classloading and symbol table population, but it also seems to
improve steady state performance of the compiler.

Theory: JIT can optimize more aggressively without the
SynchronizedXxx decorators and the like being in the classloader.
See "Class Heirarchy Analyis" in [1]

This commit does this by:

  - Avoiding use of FromString in pattern matcher, instead
    using an established mechanism to parse system properties.
  - Changes FromString back to use OptManifest. AFAICT, this
    is now only a dependency of scala.tools.cmd.gen.Codegen,
    so this is just a defensive measure.

The REPL still uses runtime reflection, so will pay a little
performance tax.

Benchmark:

    avg shortest 10 times 744ms  # before
    avg shortest 10 times 675ms  # after

[1] https://wikis.oracle.com/display/HotSpotInternals/PerformanceTechniques