Addition and subtraction ppmpf lookup tuples.

Rewired a few dependencies among the ppmpf headers, some macros from the
original alu.hh header moved into core.hh. Addition and subtraction lookup
tuples introduced.
PPMPF_IMINV(...), PPMPF_IMAXV(...) as internal ppmpf integer representations.
PPMPF_IOPC function macro added as integer overflow checkpoint.
Added selection macros for ppmpf addition and subtraction lookup tuples.
Assistive macros for ppmpf addition lookup selectors.
Addition and subtraction single digit level macro operands.
PPMPF_IADD function macro provides addition between two positive integers.

Positive integers represented as 4 member sequences of digits in the form of
(a)(b)(c)(d) and (e)(f)(g)(h) can be added using the PPMPF_IADD function
macro. Integer overflows are resolved to PPMPF_IMINV(...) and PPMPF_IMAXV(...)
macro expansions to (0)(0)(0)(0) and (9)(9)(9)(9) respectively.
PPMPF_ISUB function macro provides subtraction between two positive integers.

Positive integers represented as 4 member sequences of digits in the form of
(a)(b)(c)(d) and (e)(f)(g)(h) can be subtracted using the PPMPF_ISUB function
macro. Integer overflows are resolved to PPMPF_IMINV(...) and PPMPF_IMAXV(...)
macro expansions to (0)(0)(0)(0) and (9)(9)(9)(9) respectively.
Making PPMPF_IOPC export final representations of PPMPF_IMINV, PPMPF_IMAXV.
Less - than comparison of two positive ppmpf integers.
Comparing two positive ppmpf integers for equality.
Comparing two positive ppmpf integers for inequality.
Less than or equal, greater than or equal, greater than ppmpf comparisons.
Unit increment and decrement for positive ppmpf integers.
Explanatory text for internal ppmpf integer representation.