Implemented hash table structure for future use

Current load logic uses a linked list to read in data. That's ok for
now, but once I start loading Xresources from file, I will be stuck with
linear traversal for updating values. It doesn't seem like PuTTY uses a
hash structure anywhere I could leverage, nor are there C-implemented
hashes I like, most of which have too many features and are designed as
dynamic high-performance hashes for lots of elements. To avoid
unnecessarily increasing PuTTY footprint and complexity, I wrote my own
lightweight implementation leveraging already existing CRC32 algorithm
from SSH as hashing function.