[pdf-devel] hash helper functions

[jemarch]

Hi.

I just added two additional functions to pdf-hash-helper.h:
pdf_hash_add_size and pdf_hash_add_string.

The rule is to add new helper functions as you need it, so if you need
to insert a pdf_off_t value to a hash, please add the pdf_hash_add_off
function to pdf-hash-helper.[ch] and use it. Note that we are
supporting helper functions for types defined in pdf-types.h or
pdf-fp.h.

Gerel, what do you think about adding 'pdf_hash_get_XXX' helper
functions too? It would help to maintain the code using hashes
clear. I would go for it.