(Generated code changes in next commit.)

Generated code coming in next commit - in a subsequent PR I want to do a bit of renaming and redocumenting around this, in anticipation of DynamicMessage.

This is only thrown directly by JsonTokenizer, but surfaces from JsonParser as well. I've added doc comments to hopefully make everything clear.

The exception is actually thrown by the reader within JsonTokenizer, in anticipation of keeping track of the location within the document, but that change is not within this PR.

This includes all the well-known types except Any.
Some aspects are likely to require further work when the details of the JSON parsing expectations are hammered out in more detail. Some of these have "ignored" tests already.

Note that the choice *not* to use Json.NET was made for two reasons:
- Going from 0 dependencies to 1 dependency is a big hit, and there's not much benefit here
- Json.NET parses more leniently than we'd want; accommodating that would be nearly as much work as writing the tokenizer
This only really affects the JsonTokenizer, which could be replaced by Json.NET. The JsonParser code would be about the same length with Json.NET... but I wouldn't be as confident in it.

Rename ThrowHelper to Preconditions and make it public - we'll want to use it from the generated code soon.

Additionally, change it to return the value passed, and make it generic with a class constraint.
A separate method doesn't have the class constraint, for more unusual scenarios.

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

Everything should now be consistent - the only difference between files is the year (aside from generated files).

Fixes issue #531.

- Make some members internal
- Remove a lot of FrameworkPortability that isn't required
- Start adding documentation comments
- Remove some more group-based members
- Not passing in "the last tag read" into Read*Array, g

Remove ICodedInputStream and ICodedOutputStream, and rewrite CodedInputStream and CodedOutputStream to be specific to the binary format. If we want to support text-based formats, that can be a whole different serialization mechanism.