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The previous one was too broken, so just rewrite the code that invokes
a macro. Basically it was evaluating things too early, and mishandling
nested invocations. It's also easier to understand now.

Keep backslash-newline around in macro definitions. They get
processed when the body is evaluated.

Write some golden tests.

Change-Id: I27435f77f258a0873f80932bdc8d13ad39821ac1
Reviewed-on: https://go-review.googlesource.com/3550

Reviewed-by: Russ Cox <rsc@golang.org>