Revert "Use mrn::Field"

This reverts commit eca4384ccc7b8a3d9e88e44a35b397199adc3229.

mrn::Field isn't needed.
Revert "Create a Field wrapper"

This reverts commit 10ce5c38d4fe6f602e898221a38d7bae488c0d1f.

mrn::Field isn't needed.