Make more function parameters const

[daverodgman]

Change the public interface function parameters to be const, where appropriate.

Adding const to some internal functions is not part of this task, as it can be done at any time after 3.0.

It's recommended to submit PRs for individual modules or areas of code rather than one large PR.

[mstarzyk-mobica]

I'll take it as my first task

[mpg]

There were other github issues on the same theme, I'm linking them here for the sake of completeness:

• Introduce const qualifiers in the API wherever appropriate #1485 (links to 3 other issues + has contents of its own, some of it invalid IMO)
• mbedtls_pk_write_pubkey_der expects a mbedtls_pk_context that is non-const #396
• Constant pointers and asn1 parsing #803 (invalid IMO)

I'll comment directly on the relevant issues why I think some of them are invalid, but first I wanted to add the links here to make sure this is tracked.

[mpg]

Ok, I've now commented on the issues and PRs linked directly or indirectly from the above comment. The only one I didn't think was invalid or unwanted to other reasons was #396 for which you might want to create a PR (the existing PR addressing it also contains changes that I don't think we want, as I commented there).

[mpg]

Note: once the PRs for each module have been merged*, it would be good to add a ChangeLog entry (perhaps under "API changes") describing the changes briefly. I'm suggestion a single ChangeLog entry as this is relatively low-impact and we don't want to overwhelm users with details (especially as the 3.0 ChangeLog will contain more important things).

*Alternatively, a PR for the ChangeLog entry can be created before the other PRs are merged, but labeled "needs: preceding PR" (with references in the description) in order to reduce latency.

[gilles-peskine-arm]

@mpg Good point about changelogs. #4114

[mpg]

I think the following PRs (all merged by now) collectively resolve this issue: #4063 (aria), #4063 (asn1), #4066 (asn1write), #4102 (PK parse), #4115 (ChangeLog), unless anyone sees any more work to do here?