[ast-differ] rename variable names like valueChanged to value_diff
[ast-differ] rename variable names like valueChanged to value_diff

Summary: More idiomatic OCaml code and more accurate.

Reviewed By: nmote

Differential Revision: D10843105

fbshipit-source-id: dc0d85dff7d163b6000e9d43022b9ca21e240197

facebook/flow,popham/flow,popham/flow,jamesgpearce/flow,gabelevi/flow,facebook/flow,facebook/flow,jamesgpearce/flow,AgentME/flow,gabelevi/flow,gabelevi/flow,popham/flow,samwgoldman/flow,samwgoldman/flow,gabelevi/flow,jamesgpearce/flow,popham/flow,AgentME/flow,samwgoldman/flow,gabelevi/flow,facebook/flow,samwgoldman/flow,facebook/flow,popham/flow,AgentME/flow,mroch/flow,jamesgpearce/flow,AgentME/flow,jamesgpearce/flow,facebook/flow,mroch/flow,jamesgpearce/flow,mroch/flow,facebook/flow,mroch/flow,popham/flow,samwgoldman/flow,mroch/flow,AgentME/flow,mroch/flow,jamesgpearce/flow,samwgoldman/flow,gabelevi/flow,mroch/flow,samwgoldman/flow,gabelevi/flow,popham/flow