Currently poppler free()s/malloc()s strings a lot. Many of those re-allocations
are done for strings in Object class. Attached patch improves that by a simple
custom allocator that caches N recently free()d strings and satisfies alloc()s
from that cache, avoiding free()/malloc() cycle. In my tests the cache is very
effective: even a small cache (16 strings) can satisfy >99% of all allocations
requests.

It also changes Object::name and Object::cmd to use GooString * instead of char
*, so that they can benefit from the cache.

It also tweaks GooString and UGooString a bit and fixes a UGooString bug where
re-allocation would only preserve half of the string.

In my tests I get 2-10% speedup on loading PDF, 4% being the most common.