Draft: Less tzresult, more result

Context

Existing code such as the one discussed in !2765 (comment 555727601) shows that it's not trivial to manage different kinds of errors.

This MR makes more basic functions (the ones in stdlib) return more basic kinds of results (('a, exn) result). Currently, these functions are wrapped at the call-site to transform into less-basic kinds of results ('a tzresult). But the idea is to allow to only use tzresult when needed.

Ping @rafoo_ @sophiagold who are involved in the conversation mentioned above.

Manually testing the MR

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.