Make FilesAnalyzed a pointer, assume "true" if absent

[ianling]

Closes #145

Depends on and contains commits from #134, #135, #137, #138, and #139

Per the spec, FilesAnalyzed is not required to be present. If it is not present, the value is assumed to be true.

This was previously handled using a secondary field that denoted whether or not FilesAnalyzed was present.

This PR removes that field and makes FilesAnalyzed a pointer to denote that it is not required to be present.

All of the parsers interpret an absence of a value as true.

All of the file writers will ALWAYS write a value for FilesAnalyzed, either true or false, they will never omit it. I think it is better for SPDX files to be explicit.

[kzantow]

Hi @ianling -- there was a significant refactoring in this library, are you able to adjust this PR for the changes? Also, you will need to sign-off your commits

[kzantow]

Hiya @ianling -- thank you for this (and the other) contributions. Since this is dependent on the others, I'm going to close it for now. We will gladly reopen this if you decide to update/continue it, but I think it also may have been superseded by #210