Merge changes from my fork back to main Repo

[oformaniuk]

What's inside (inspired by /issues/294):

• fixes to multiple Github issues:
  • BlockParams support (Sequence contains more than one element #301)
  • Support returning objects from helpers (Support for returning a complex object from a subexpression #264)
  • Lookup helper (Cannot compile template with Lookup helper - signature or security transparency is not compatible #268)
  • JObject performance problems (rexm/issues/308)
  • Support more advanced helper construction (Question: How to create a custom helper with access to @index #151)
  • Support helper resolvers ([Suggestion] Fallback to instance/static method if helper cannot be resolved #336)
  • Other issues
• global refactoring (without breaking changes to the public API)
  • use zjklee/ExpressionShortcuts to improve maintainability of Expression Trees
• improvements to extensibility
• performance improvements (depends on template complexity):
    - 2-10x compilation time improvement 
    - 2-10x execution time improvement 
    - up to 20x less memory consumption during compilation and execution

Due to the nature and amount of changes I'm not able to extract them to a separate PRs one for each issue at this point.
I'd continue to support my fork (and back-port changes here) until the PR is merged.

[jrlost]

Can we get more eyes on this? @zjklee your performance enhancements alone warrant a strong consideration for merging this. We are actively using the fork on some high traffic sites and have seen massive improvements to compile time and allocations (and the related reduction in heap size and GCs). I'm talking orders of magnitude improvements. I cannot personally speak to any of the other functionality changes as we're not actively using them, but the compile and runtime improvements are great.

My only issue is that I'm forced to pull a separate nuget repository to run it, which has a separate token, so I cannot just update in-place; I'm forced to drop this repos package, add the package from the fork, update references, etc... Then once this gets merged, if it does, I need to undo all of that.

[Badabum]

@rexm any chance you'll have time to take a look?