Implement and test hash types

[spk121]

Implement scheme-to-GHashTable conversion functions and ensure that all the libmarshall tests pass for hash tables.

[spk121]

So it appears that in the GIR and Typelib files, all of the GHashTable constructors are marked as not introspectable, so there is no way to construct a GHashTable via introspected methods. However, the type itself and many methods are introspectable.
Thus, any GHashTable constructors will need to be supplied by this project.

[spk121]

I've implemented a minimal interface. It is primitive, but, it provides the minimum functionality and tracks the C API. I'm not sure about what higher-level API would be appropriate, since so much depends on interpreting the pointers that GHashTables use as their keys and values.

[spk121]

These patches always convert a Guile hashtable to a GHashTable and vice versa as required by a function call. OUT and INOUT are handled correctly. The next problem is that a GHashTable can be marked as IN only in a function call, but, the function can still modify the hashtable. (see g_hash_table_insert)
Two strategies

• Return a Guile representation of the GHashTable as output, as if it were an INOUT parameter. (But can't use INOUT handling directly because of the extra level of indirection of INOUT parameters)
• After the function call, modify the Guile hashtable to match the GHashTable