Add m.prop

[dead-claudia]

Description

Exactly what it says on the tin, but I added the .get/.set version proposed in this comment

In addition to what was proposed there, I also added .toJSON as an alias for .get.

Motivation and Context

Fixes #2095

How Has This Been Tested?

Added a test just to make sure it works. (I'm not fully convinced it was worth the time to write and run the test, though.)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/change-log.md

[StephanHoyer]

Where is the implementation of m.prop? Forgot to add?

[dead-claudia]

@StephanHoyer https://github.com/MithrilJS/mithril.js/pull/2268/files#diff-5a33bbbbbbb77492a6d4259fa1b2578e

99% of the diff isn't actually code, but docs updates. m.prop really is that small. 😉

[StephanHoyer]

Oh, snap.

LGTM! Although I liked the overloaded nature of the old m.prop. But I'm ok with it.

[dead-claudia]

There was some discussion in #2095 suggesting separate get/set functions instead of a single overloaded closure. It's easier to compose and easier to differentiate reads from writes, which is helpful when the prop isn't defined locally.

BTW, this is mostly a port of this minus the observation abilities.