Add an .editorconfig file

Just a start. Could help enforcing the Coding Style
https://github.com/Microsoft/dotnet/blob/master/CONTRIBUTING.md#c-coding-style

What is .editorconfig:
http://editorconfig.org/#overview

Plugins for IDEs to support:
http://editorconfig.org/#download
Added trim_trailing_whitespace = true
Add optimization for ImmutableList<T>.AddRange when adding non-empty range
to non-empty list
Apply same optimization for ImmutableList<T>.InsertRange, also add more
  test coverage
Make InsertRange rebalance list correctly
Change Node.Root to internal to facilitate testing
Revert changes to InsertRangeTest
Add more tests
Add doc comment for BalanceNode method
Add reference equality check to SetEquals methods.

The SetEquals methods are rather expensive, and it's a quick and super-effective check to compare reference equality between the two collections before engaging in the expensive work.
Remove use of non ASCII characters in literals.

We would prefer that we don't use non ASCII characters in either string
or character literals.  In theory, every editor and tool would ensure
that these characters are not translated to some random encoding or
otherwise become corrupted.  In practice, we're just going to err on the
side of caution.
Replace non ASCII characters in comments with ASCII equivalents.
Merge branch 'remove-unicode-characters'
Add more verifications to ImmutableList.InsertRange perf fix.

InsertRangeRandomBalanceTest currently fails on all of my runs because VerifyBalanced finds height differences of 2 or -2.
One failing run is with rand seed: 1135977305
Fix links to blog posts
Add more verifications to ImmutableList.InsertRange perf fix.

InsertRangeRandomBalanceTest currently fails on all of my runs because VerifyBalanced finds height differences of 2 or -2.
One failing run is with rand seed: 1135977305
fixing issue #99
Call AddRange and InsertRange from corresponding Builder methods
 - This fixes failing InsertRangeRandomBalanceTest
Also refactor AddRangeBalanceTest to add random batches and verify
  balance after each
Removed internal TFS bindings from System.Reflection.Metadata.sln
Merge branch 'patch-1' of https://github.com/Blaisorblade/corefx
Make test failures easier to parse.
Fix ImmutableList`1+Node.MakeBalanced method.

It wasn't balancing the tree properly, as it would allow branches to be +/-2 off instead of only allowing a variance of +/-1. This was caught by the InsertRangeRandomBalanceTest failing test, which now passes.
Move VerifyBalanced method to a more common test location.

This will hopefully allow us to use it for other AVL-based collection types.