[m-rev.] for review: reserved address data representation

[Fergus Henderson]

Estimated hours taken: 40
Branches: main

Allow the compiler to optionally make use of reserved addresses --
null pointers, ints cast to pointers, and addresses of global variables --
to optimize the representation of constants in discriminated union types.

This will be particularly useful for the .NET and Java back-ends.

There are several parts to this change:
	- compiler support
	- RTTI support
	- fixes for code which makes assumptions that no longer hold
	- optimizing trivial switches in the MLDS back-end

For ease of review, I will post each of these parts separately.

-- 
Fergus Henderson <fjh at cs.mu.oz.au>  | "... it seems to me that 15 years of
The University of Melbourne         | email is plenty for one lifetime."
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- Prof. Donald E. Knuth
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