The types used while generating code, such as Rule and File, have been
exported for a while. This is harmless for a main package, and lets us
easily differentiate types from variables and functions, as well as use
names like "If" since "if" is a keyword.
However, the fields remained unexported. This was a bit inconsistent,
and also meant that we couldn't use some intuitive names like If.else.
Export them.
Besides the capitalization, the only change is that the If type now has
the fields Then and Else, instead of stmt and alt.