Update Morph to latest version.

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https://github.com/tildeio/htmlbars/compare/1a4d5985824f4df23bb6e78829e442fe368937aa...7889d3f4d2a979f8e32648c92afa033f1f32b434

This includes some perf fixes (in https://github.com/tildeio/htmlbars/pull/79) by Stef to allow Morph's constructor to be inlineable.