Re: new heapcheck contrib module

I think there are two very large patches here. One adds checking of
heapam tables to amcheck, and the other adds a binary that eases calling
amcheck from the command line. I think these should be two separate
patches.

I don't know what to think of a module contrib/pg_amcheck. I kinda lean
towards fitting it in src/bin/scripts rather than as a contrib module.
However, it seems a bit weird that it depends on a contrib module.
Maybe amcheck should not be a contrib module at all but rather a new
extension in src/extensions/ that is compiled and installed (in the
filesystem, not in databases) by default.

I strongly agree with hardening backend code so that all the crashes
that Mark has found can be repaired. (We discussed this topic
before[1]: we'd repair all crashes when run with production code, not
all assertion crashes.)

[1] https://postgr.es/m/20200513221051.GA26592@alvherre.pgsql

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Álvaro Herrera https://www.2ndQuadrant.com/
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