Implement new string readers and writers for reader/memreader/writer/mwriter

[katajakasa]

Since libshadowdive is now part of openomf, we could start using the string wrapper in libsd operations too. To begin with, we should add these functions to the reader/memreader/writer/memwriter to make operating on strings easier.

To writer/memwriter:

• Add function to write an omf2097 null padded string type from str object
• Add function to write an omf2097 not-padded string type from str object

To reader/memreader:

• Add function to read an omf2097 null padded string type to str object
• Add function to read an omf2097 not-padded string type to str object

This thing should really have unit-tests also. Perhaps create a static test file that contains data known good (check with hex editor) and then test reader against that. Then write a second file with writer, and test that produced file matches the existing test file bit-for-bit.

There is some ready made code here: https://github.com/omf2097/openomf/pull/364/files -- it is not tested though.