make string-append on win32 100 times faster

make string-append on win32 100 times faster

When a string grows (e.g. gets appended to), perl calls sv_grow. When
sv_grow decides that the memory currently allocated to the string is
insufficient, it calls saferealloc. Depending on whether or not perl
was compiled with 'usemymalloc' this calls realloc in either perls
internal version or on the operating system. Perl requests from
realloc just the amount of memory required for the current
operation. With 'usemymalloc' this is not a problem because it rounds
up memory allocation to a certain geometric progression anyway. When
the operating system's realloc is called, this may or may not lead to
desaster, depending on how it's implemented. On win32 it does lead to
desaster: when I loop 1000 times and each time append 1000 chars to an
initial string size of 10 million, the memory grows from 10.000e6 to
10.001e6 to 10.002e6 and so on 1000 times till it ends at 11.000e6.