Maint: Add an assertion mechanism to Puppet

This patch allows us to make C-style "assertions" in Puppet code,
e.g.:

assert_that { condition }
assert_that(message) { condition }

These methods will raise an exception if the environment variable
PUPPET_ENABLE_ASSERTIONS is set to a non-empty value, and the the
condition evaluates to false.  If the environment variable
PUPPET_ENABLE_ASSERTIONS is not set, then the condition is not even
checked.

Switching the assertions on with PUPPET_ENABLE_ASSERTIONS carries
three advantages:

1. It makes it possible to put potentially expensive checks in
assertions without degrading the performance of the code in production
environments.

2. It allows strict assertions to catch Puppet bugs early in
development, without increasing the risk of a crash in production
environments.

3. It allows a simple command-line mechanism to run any Puppet command
with assertions enabled.

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