Parse attribute-style comments from source code

[richfitz]

This is the first stage of supporting compiling models; parsing some metadata out of the source code. See the new attributes in inst/examples/sir.cpp and examples/walk.cpp for examples of this.

When we come to do code generation we will not attempt to parse C++ (which is hard) but instead inspect these attributes and generate code based on what we find.  This is what we ended up doing in dust1 eventually, it's much easier than trying to be clever, and practically it's not very hard to do because most models are generated mechanically anyway.

At the same time I've updated the parameter- and data-reading functions to use some common code, which provides much nicer error messages.

We'll probably expand some of the attribute support here, in particular advertising parameter types and if the parameter is updateble, differentiable, or has a default. For now the bare minimum.

The error reporting could be improved and it would not be that hard to return line numbers etc. But the expectation is that most of the code is generated so these errors will probably never be seen

[weshinsley]

Great - this looks nice

[weshinsley]

That mustWork arg name always amuses me. I might write some args not_that_bothered_if_it_works in some of my functions.