Update Morph to latest version.

Compare view:

https://github.com/tildeio/htmlbars/compare/1a4d5985824f4df23bb6e78829e442fe368937aa...7889d3f4d2a979f8e32648c92afa033f1f32b434

This includes some perf fixes by Stef to allow Morph's constructor to be
inlineable.
Merge pull request #5448 from rwjblue/update-morph

Update Morph to latest version.