Refactor reader and writer functions

[katajakasa]

Libshadowdive had its internal memreader/reader/memwriter/writer functions in internal/ subdirectory (formats/internal/). We should move those to the openomf projects general utility directory (utils/) and refactor them to match openomf coding style.

• Remove "sd_" prefix from functions
• Rename mwriter to memwriter in function and struct names (etc).
• Rename mreader to memreader in functio nand struct names (etc).
• Drop the pointless struct names in all of these readers and writers -- just keep the typedeffed name.
• Remove the "SD_" prefix from guards, and make sure the guards match the filenames
• Add consts where possible
• Check that docs are in order, and add some new ones if possible.

Verify afterwards:

• Everything compiles
• Unit-tests
• Game still starts :)
• Utilities (aftool, bktool, ... ) should still work