Rewrite gnc-numeric to be multi-precision decimal float

Using the decimal float library from http://www.bytereef.org/mpdecimal
Adjust Engine files to compile with mpdecimal

Not to be confused with actually working!
Adjust gnome, gnome-utils, and gnome-search for mpdecimal
Adjust app-utils for mpdecimal
Adjust backends, business, and engine and qof tests to compile with mpdecimal.
Adjust import-export to compile with mpdecimal
Initial changes to gnc-numeric tests for mpdecimal.

Only about half pass at this point, and it still crashes partway through.
Remove qofmath128
Fix kvp_frame numeric string tests to remove rational number notation for gnc_numeric_zero values.
Provide a fast (comparisons only) function to find the exponent value of an int64_t.
Fix attempt to assign gnc_numeric from 2-element array.
Declare const member functions which don't change the object.

So that they can be called on const objects.
Change signature of GncNumeric::apply_binary to add denom argument.

So that it rounds correctly when denom is used as part of the rounding spec.
Whitespace fixups.
Add default_round constant for clarity and uniformity.
Round operands to equal exponents and digits in computations.

To lessen the likelihood of rounding discrepancies when comparing results.
Implement two-stage fuzzy compare.

Necessary to work around rounding discrepancies when dealing with
high-significant-figure results; these are inevitable when handling numbers
like 1/3 which do not express exactly in decimal.
Ensure that a GncNumeric is restored to having static data after a computation.

Sometimes a computation will temporarily overflow the static allocation and switch to
using dynamic memory. This is good because it gracefully handles problem computations,
but doesn't work so well with our emphasis on stack-allocated numerics, so ensure that
when it happens the data is transferred back to the stack-allocated buffer and the static-data
flag is reset.
Remove pre-rounding of binary function operands.

Not needed once dynamic allocations are restored to static.
Replace most uses of the IEEE_728_128_BIT constants

Abstract for easier tuning.