Task: Upgrade codebase to use BufferHelper instead of conditionally compiling with System.Buffer

[NightOwl888]

System.Buffer is not implemented prior to net46, but we have code that requires both its public and internal methods. A new internal class, J2N.BufferHelper is being implemented in the fix for #117.

We need to migrate all BCL code that uses System.Buffer to J2N.BufferHelper. However, do note that there are no tests for this class, so we will need to rely on it not causing other tests to fail. So, careful attention needs to be paid when switching to each method that none of the tests start failing (both in 32 and 64 bit tests).

[paulirwin]

Thoughts on dropping the net40 and net45 targets in a major version bump instead? All .NET Framework 4.x versions prior to 4.6.2 have been out of support for at least a few years now.

[NightOwl888]

Add .NETFramework 4.0 support was submitted by one of our sponsors. They specifically requested that ICU4N have net40 support even though it was already out of support at that time. Given the fact that there has not yet been an RTM release of ICU4N on net40, it seems like doing so would leave them holding the bag.

Good technical decision? No. Good financial decision? Yes.

That being said, I see no reason to support net40 anywhere else.

Most of the methods in BufferHelper were internal in the BCL, anyway. This just isolates them and makes them easier to optimize in one place.