Merge C version of decimal into py3k.

Modules/_decimal/ISSUES.txt

+
+
+Normal priority:
+----------------
+
+1) Add C-API importable as capsule.
+
+
+Low priority:
+-------------
+
+1) Convert tabs (wait until commit).
+
+2) Pre-ANSI compilers require '#' in the first column (should be done
+   for the whole Python source tree if we support such compilers). (?)
+
+3) FETCH_CURRENT_CONTEXT vs. CURRENT_CONTEXT?
+
+4) Justify remaining uses of exit on overflow in bignum arith. Short
+   answer: with correct context values the coefficients never get big
+   enough for that to happen.
+
+5) Justify remaining uses of abort() in mpdecimal: These uses are
+   for debug purposes and can't be reached when the library is used
+   correctly.
+
+