Fix Xcode warnings, comments

[christophhagen]

There are several warnings due to "unsafe" type conversions and invalid comments when importing libsignal-protocol-c into an Xcode project.

This pull request fixes these warnings, without changing any functionality.

[dkonigsberg]

Could you please split this one up into several pull requests? It looks to me like its a combination of 3 distinct categories of changes:

• Fixes to comments
• Fixes for warnings from non-curve25519 code
• Fixes for warnings to (external'ish) curve25519 code

The first two I'm likely to merge with little hesitation, and would like to get in sooner rather than later.

[christophhagen]

I made the changes as requested, closing this PR.