Completing several ppmpf constructs and constructors.

This merge introduces ppmpf tuple constructors and sequences, safe and unsafe
modalities, deprecates old placeholder semantics, upgrades recursive expansion
constructs and deals with several design issues in ppmpf.
Merging several changes to tests into the master branch.

Several tests have been split into distinct files, test macros expand
only when 'make test' is issued, 'make tests' functionality merged into
'make test' target. The whole format for ppmpf tests allows for easy
test generation.
Updated readme files for the -DUSE_TEST_EXPANSION flag.
Periodic synchronization of devbox with the master branch.