Add CMap caching at document level and add MurmurHash3 hashing function

[BobLd]

Cache custom CMaps at document level (not shared across documents) in order to avoid re-parsing existing CMap. Also avoid keeping duplicate CMaps in memory.

Custom CMaps with the same name can represent different data. To ensure CMaps are the same before parsing them, we use hashing. MurmurHash3 is used.

This cache is on top of the existing cache for common CMaps (shared across documents).

Note on MurmurHash3:

• MurmurHash is a non-cryptographic hash function suitable for general hash-based lookup (https://en.wikipedia.org/wiki/MurmurHash)
• It is used to hash CMap byte arrays to identify if they are the same.
• I started with the MD5 function, but the methods are not available on browser. MurmurHash3 is also supposed to be faster
• The implementation is a port from C++ to C# https://github.com/aappleby/smhasher/blob/master/src/MurmurHash3.cpp The code is "placed in the public domain." by the author - see link.