Add variant_under_utf8_count() core function

Add variant_under_utf8_count() core function

This function takes a string that isn't encoded in UTF-8 (hence is
assumed to be in Latin1), and counts how many of the bytes therein
would change if it were to be translated into UTF-8.  Each such byte
would occupy two UTF-8 bytes.

This function is useful for calculating the expansion factor precisely
when converting to UTF-8, so as to know how much to malloc.

This function uses a non-obvious method to do the calculations
word-at-a-time, as opposed to the byte-at-a-time method used now, and
hence should be much faster than the current methods.

The performance change in short string lengths is equivocal.  Here is
the result for a single character and a 64-bit word.

          bytes    words Ratio %
        -------- -------- -------
     Ir    932.0    947.0    98.4
     Dr    325.0    325.0   100.0
     Dw    104.0    104.0   100.0
   COND    136.0    137.0    99.3
    IND     28.0     28.0   100.0

 COND_m      1.0      0.0   Inf
  IND_m      6.0      6.0   100.0

There are some extra instructions executed and an extra branch to check
for and handle the case where we can go word-by-word vs. not.  But the
one cache miss is removed.

The results are essentially the same until we get to being able to
handle a full word.  Some of the extra instructions are to ensure that
if the input is not aligned on a word boundary, that performance doesn't
suffer.

Here's the results for 8-bytes on a 64-bit system.

           bytes    words Ratio %
        -------- -------- -------
     Ir    974.0    955.0   102.0
     Dr    332.0    325.0   102.2
     Dw    104.0    104.0   100.0
   COND    143.0    138.0   103.6
    IND     28.0     28.0   100.0

 COND_m      1.0      0.0     Inf
  IND_m      6.0      6.0   100.0

Things keep improving as the strings get longer.  Here's for 24 bytes.

           bytes    words Ratio %
        -------- -------- -------
     Ir   1070.0    975.0   109.7
     Dr    348.0    327.0   106.4
     Dw    104.0    104.0   100.0
   COND    159.0    140.0   113.6
    IND     28.0     28.0   100.0

 COND_m      2.0      0.0     Inf
  IND_m      6.0      6.0   100.0

And 96:

           bytes    words Ratio %
        -------- -------- -------
     Ir   1502.0   1065.0   141.0
     Dr    420.0    336.0   125.0
     Dw    104.0    104.0   100.0
   COND    231.0    149.0   155.0
    IND     28.0     28.0   100.0

 COND_m      2.0      1.0   200.0
  IND_m      6.0      6.0   100.0

And 10,000

           bytes    words Ratio %
        -------- -------- -------
     Ir  60926.0  13445.0   453.1
     Dr  10324.0   1574.0   655.9
     Dw    104.0    104.0   100.0
   COND  10135.0   1387.0   730.7
    IND     28.0     28.0   100.0

 COND_m      2.0      1.0   200.0
  IND_m      6.0      6.0   100.0

I found this trick on the internet many years ago, but I can't seem to
find it again to give them credit.