Add a cautionary comment to TreeSymSubstitutor.
Refactor existential related code out of types.

For imminent reuse in the subsequent commit.
Simplify interplay between Uncurry Info- and Tree-Transformers

Now, the InfoTransformer is responsible for erasing the
path dependent types of formal parameters, at the same
place where it flattens nested method types.

This is preferable to having the tree transformer overwrite
the result of the info transformer, as used to be the case
after my previous work on SI-6135 / 493197fc.
SI-6900 Fix tailrec for dependent method types.

Uncurry's info transformer could generate a MethodType
with cloned parameter symbols. This type was used
for the LabelDef generated in the TailCalls phase.
But, the RHS of the method still contains types that
refer to the original parmameter symbol. Spurious,
type errors ensued.

This commit changes Uncurry's tree transformer to
keep the symbols in the DefDef consistent with the
results of the InfoTransformer.