Resolve compiler errors and linter warnings

[pjones]

Allow this package to be byte compiled by resolving compiler errors. Also addresses all warnings from package-lint.

[nicolas-graves]

I still do have one error after this patch:

In end of data:
persid.el:530:35: Warning: the function ‘json-parse-buffer’ is not known to be defined.

[rougier]

Back to work, sorry for long long delay. There are now some conflicts, could you possibly rebase ? Else I can probably manually resolve conflicts.

[pjones]

I will rebase (hopefully) some time this week.

[pjones]

I rebased this PR.

Note: package-lint-current-buffer still reports the following issues but I didn't want to make larger changes than I already have.

5 issues found:

142:1: warning: `eval-after-load' is for use in configurations, and should rarely be used in packages.
510:0: error: `persid--openalex/normalize-name' contains a non-standard separator `/', use hyphens instead (see Elisp Coding Conventions).
518:0: error: `persid--openalex/venue' contains a non-standard separator `/', use hyphens instead (see Elisp Coding Conventions).
535:0: error: `persid--pmc/pmid-to-doi' contains a non-standard separator `/', use hyphens instead (see Elisp Coding Conventions).
549:0: error: `persid--pmc/pmcid-to-doi' contains a non-standard separator `/', use hyphens instead (see Elisp Coding Conventions).