Fix warnings in mcs project and its dependencies

Fixed a bunch of warnings and enable the "Treat warnings as errors" flag in the projects that are now warning free. The only left with warnings is pscorlib_aot-net_monomac due to a bug see issue https://github.com/playscript/playscript-mono/issues/243 for details in the bug. Made a bunch of small commits to help review the changes.

**Do not merge this pull request into master until 'aggressive-inlining' branch has been merged into master.**