Related to #658 and #5007.

Add preserve UnknownFields
Compare floating point values bitwise
Add auto-generated header to C# generated files

This consists of:
- Changing the codegen for the fixed set of options protos, to parse unknown fields instead of skipping them
- Add a new CustomOptions type in the C# support library
- Expose CustomOptions properties from the immutable proto wrappers in the support library

Only single-value options are currently supported, and fetching options values requires getting the type right
and knowing the field number. Both of these can be addressed at a later time.

Fixes #2143, at least as a first pass.

JSON tests fail, as we're not using OriginalNameAttribute yet.

There are corner cases where MessageDescriptor.{ClrType,Parser} will return null, and these are now documented. However, normally they *should* be implemented, even for descriptors of for dynamic messages. Ditto FieldDescriptor.Accessor.
We'll still need a fair amount of work to implement dynamic messages, but this change means that the public API will be remain intact.

Additionally, this change starts making use of C# 6 features in the files that it touches. This is far from exhaustive, and later PRs will have more.

Generated code changes coming in the next commit.

Biting off just this bit first as I don't need the changes from a previous PR for this part.

This is a bit of a grotty hack, as we need to sort of fake proto2 field presence, but with only a proto3 version of the descriptor messages (a bit like oneof detection).
Should be okay, but will need to be careful of this if we ever implement proto2.

(Shows the benefit of unit testing even code "too simple to fail"...)

I think Jan was actually suggesting keeping both, but that feels redundant to me. The test diff is misleading here IMO, because I wouldn't expect real code using reflection to use several accessors one after another like this, unless it was within a loop. Evidence to the contrary would be welcome :)

This change also incidentally goes part way to fixing the issue of the JSON formatter not writing out the fields in field number order - with this change, it does except for oneofs, which we can fix in a follow-up change.

I haven't actually added a test with a message with fields deliberately out of order - I'm happy to do so though. It feels like it would make sense to be in google/src/protobuf, but it's not entirely clear what the rules of engagement are for adding new messages there. (unittest_proto3.proto?)

This is definitely not ready to ship - I'm "troubled" by the disconnect between a list of fields in declaration order, and a mapping of field accessors by field number/name. Discussion required, but I find that easier when we've got code to look at :)

Changes in brief:
1. Descriptor is now the entry point for all reflection.
2. IReflectedMessage has gone; there's now a Descriptor property in IMessage, which is explicitly implemented (due to the static property).
3. FieldAccessorTable has gone away
4. IFieldAccessor and OneofFieldAccessor still exist; we *could* put the functionality straight into FieldDescriptor and OneofDescriptor... I'm unsure about that.
5. There's a temporary property MessageDescriptor.FieldAccessorsByFieldNumber to make the test changes small - we probably want this to go away
6. Discovery for delegates is now via attributes applied to properties and the Clear method of a oneof

I'm happy with 1-3.
4 I'm unsure about - feedback welcome.
5 will go away
6 I'm unsure about, both in design and implementation. Should we have a ProtobufMessageAttribute too? Should we find all the relevant attributes in MessageDescriptor and pass them down, to avoid an O(N^2) scenario?

Generated code changes coming in the next commit.

We'll see what I've missed when CI fails...