method transformClassTemplate() turned into class TemplateTransformer

Before this commit, each activation of transformClassTemplate()
performed the rewriting that the constructors phase is well-known for.
Lots of auxialiary objects were created in the process,
with lifetime confined to said activation.

The same auxiliary objects (having same lifetimes as before)
also are in effect starting with this commit,
but now it's an instance of TemplateTransformer
that holds them together. In other words,
there's a one-to-one correspondence between:
- (what used to be) transformClassTemplate() activation
- TemplateTransformer initialization

After initialization, the result of TemplateTransformer
can be found in its `transformed` member val.

In fact, the refactoring to get here from the previous commit
basically involves taking the body of method transformClassTemplate()
as-is to become the template of TemplateTransformer.

The TemplateTransformer in question will allow modularizing
sub-transformations (e.g., DelayedInit) into dedicated traits
(see  upcoming commits).
separation of concerns: delayed-init in constructors

This commit modularizes TemplateTransformer by moving a few methods
from TemplateTransformer into the newly added DelayedInitHelper.

The methods in question
- delayedInitCall()
- delayedInitClosure()
- delayedEndpointDef()

build trees that rewriteDelayedInit() puts together.
That way, all trace of rewriting related to delayed-init
have vanished from TemplateTransformer and are encapsulated in
DelayedInitHelper.
separation of concerns: eliding param-accessor fields in constructors

This commit modularizes TemplateTransformer by moving a few members
from TemplateTransformer into the newly added OmittablesHelper.

The members in question include

- a few methods:
  # isParamCandidateForElision(Symbol)
  # isOuterCandidateForElision(Symbol)
  # mustbeKept(Symbol)

- a few vals:
  # paramCandidatesForElision
  # outerCandidatesForElision
  # bodyOfOuterAccessor

- class UsagesDetector

That way, all trace of rewriting to elide param-accessor fields
has vanished from TemplateTransformer and is encapsulated in
OmittablesHelper.