Optimizations in FcStrSet

Applied optimizations:
- skip duplicate check in FcStrSetAppend for values originating from readdir()
- grow FcStrSet in 64-element bulks for local FcStrSets (FcConfig layout unaltered)

Starting gedit is measured to

                        Unoptimized     Optimized
user[s]                         0,806         0,579
sys[s]                          0,062         0,062
Total Instr Fetch Cost: 1.658.683.750   895.069.820
Cachegrind D Refs:        513.917.619   312.000.436
Cachegrind Dl Misses:       8.605.632     4.954.639
6 files changed