Synchronizing with master branch (readme fixes etc).
Added macros for ppmpf hexadecimal recursion depth maximum.

 *) PPMPF_IEXEX: converts internal hex representation to 0xNNNN
 *) PPMPF_RDMAX: if not set, defaults to 65535 -> (15)(15)(15)(15) -> 0xFFFF
 *) PPMPF_RDMH4(), PMPF_RDMH3(), PPMPF_RMDH2(), PPMPF_RDMH1() are the internal
    "digit" representations for hexacedimal recursion depth numbers.

NB: PPMPF_RDMAX() is derived from PPMPF_RDMAX as defined during compilation to be
    a decimal number (0 - 65535).
Making PPMPF_FLDX0G dependent of PPMPF_RDMAX predefined depth.
Copyright year fix in <odreex/ppmpf/algorithm/fldaux.hh>

Much of the code introduced in this header was excised from its fold.hh
counterpart during refactoring; fldaux.hh in this case is thus derived
from the 2013 codebase in odreex/ppmpf and therefore this change is
justified.
Towards taking care of safe / unsafe ppmpf tuples.

A rather complicated commit, aiming towards deprecating the older recursive
expansion macro construct. Safe and unsafe ppmpf tuples have separate fold,
map, zip, zipwith high order macros. The addition of recursively expanding
"double" folding construct targets self - reentrant issues when multiple
"higher order" macro functions are combined with ppmpf tuples.
Cleaning up previous fold implementations in ppmpf.
Several changes to ppmpf sequences, resolves #1

The new recursive expansion construct is used; folding, mapping and to tuple
transformation fixed accordingly.
Added missing tests for ppmpf sequence folds.
Added missing test for ppmpf sequence constructor.
Merging recursive expansion consruct refactoring, ppmpf collections' change.
Added ppmpf sequence constructor, resolves #15.
Adjusted ppmpf nrange for safe tuples and new recursion mode.
Dealing with several issues related to the devbox branch, fixes #16.

A series of bugs related to transitioning to the newer recursive expansion
macro construct and a major regression noted in zip / zipwith tests have
been addressed. In summary:

    *) Deprecation of old placeholder semantics is now valid for the new
       incarnations of high order metafunctions.
    *) Composition macro has been adjusted for the same construct.
    *) The range generator macros have been properly rewired for the same
       construct.
    *) More tests were added.

The actual resolution of #16 would not have been possible but by also avoiding
artificial macro reentrancies during expansion created by the over - reuse of
metafunction macros. This is another reason why metafunction "doubles" (same
final expansion, different macro identifier) must exist in constructs that
are highly prone to nesting one with the other during emulation of recursion,
because of their code - scaffolding properties in recursive expansion designs.
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.