Print message before trapping, on DFINITY mode

[nomeata]

previously, in a large number of possible error conditions, whether they
are benign (array index out of bounds) or internal (type error,
unimplemented code) we just called Unreachable, causing a trap. But
this is really not helpful to our early adopters.

So in DFINITY mode, before calling Unreachable, print a helpful
error message via test.print.

I made the strings all static, so that multiple calls with the same
error message do not have to allocate static spaces multiple times. (It
currently still does, but making sure every static string is only added
to memory once is easy to do, and independent of this change.)

Some cleanup of compile_lit along the way. Also I prefer let open Foo in over Foo.(…) when … is a multi-line expression with control flow
and it is the last expression of the current function anyways.