Overallocate for speed in str.c

Overallocate for speed in str.c

One of the key motivations for switching to the str_t type internally
is that we can avoid allocations by re-using the same storage over and
over during the transformation.

We can avoid other allocations by overallocating when more storage
is requested, seeing as almost all requests for more storage will
later be followed by other requests.

At the moment, the original implementation is quite fast:

                                user     system      total        real
short slab of ASCII text    1.440000   0.000000   1.440000 (  1.445547)
short slab of UTF-8 text    2.900000   0.010000   2.910000 (  2.927274)
longer slab of ASCII text  12.710000   0.040000  12.750000 ( 12.816209)
longer slab of UTF-8 text  35.210000   0.080000  35.290000 ( 35.661577)

The new implementation is actually slower, because we have had to add
some wasteful conversions back-and-forth between VALUE/String and str_t:

short slab of ASCII text    1.550000   0.000000   1.550000 (  1.556956)
short slab of UTF-8 text    3.340000   0.010000   3.350000 (  3.377874)
longer slab of ASCII text  15.410000   0.030000  15.440000 ( 15.484308)
longer slab of UTF-8 text  45.230000   0.130000  45.360000 ( 45.631355)

It is expected that the performance loss will be recovered once these
wasteful conversions are eliminated.

But before going that far, adding overallocation brings very large
improvements, enough to compensate for the inefficient conversions:

short slab of ASCII text    1.190000   0.010000   1.200000 (  1.233443)
short slab of UTF-8 text    2.460000   0.020000   2.480000 (  2.536714)
longer slab of ASCII text  11.000000   0.050000  11.050000 ( 11.208843)
longer slab of UTF-8 text  33.490000   0.130000  33.620000 ( 34.190941)

Those numbers use an overallocation constant of 256 bytes; will later
experiment with other constants to find the optimal overallocation.

Signed-off-by: Wincent Colaiuta <win@wincent.com>