Add a new function, gdb.format_address, which is a wrapper around
GDB's print_address function.

This method takes an address, and returns a string with the format:

  ADDRESS <SYMBOL+OFFSET>

Where, ADDRESS is the original address, formatted as hexadecimal,
SYMBOL is a symbol with an address lower than ADDRESS, and OFFSET is
the offset from SYMBOL to ADDRESS in decimal.

If there's no SYMBOL suitably close to ADDRESS then the
<SYMBOL+OFFSET> part is not included.

This is useful if a user wants to write a Python script that
pretty-prints addresses, the user no longer needs to do manual symbol
lookup, or worry about correctly formatting addresses.

Additionally, there are some settings that effect how GDB picks
SYMBOL, and whether the file name and line number should be included
with the SYMBOL name, the gdb.format_address function ensures that the
users Python script also benefits from these settings.

The gdb.format_address by default selects SYMBOL from the current
inferiors program space, and address is formatted using the
architecture for the current inferior.  However, a user can also
explicitly pass a program space and architecture like this:

  gdb.format_address(ADDRESS, PROGRAM_SPACE, ARCHITECTURE)

In order to format an address for a different inferior.

Notes on the implementation:

In py-arch.c I extended arch_object_to_gdbarch to add an assertion for
the type of the PyObject being worked on.  Prior to this commit all
uses of arch_object_to_gdbarch were guaranteed to pass this function a
gdb.Architecture object, but, with this commit, this might not be the
case.

So, with this commit I've made it a requirement that the PyObject be a
gdb.Architecture, and this is checked with the assert.  And in order
that callers from other files can check if they have a
gdb.Architecture object, I've added the new function
gdbpy_is_architecture.

In py-progspace.c I've added two new function, the first
progspace_object_to_program_space, converts a PyObject of type
gdb.Progspace to the associated program_space pointer, and
gdbpy_is_progspace checks if a PyObject is a gdb.Progspace or not.