Provide a nil safe dereference operator

[hopscc]

Provide an operator that will allow deref through a chain of calls and member accesses even if something in the chain returns nil.
(nilsafe/elvis operator)

Like DOT operator but returns nil if prior element return nil
See  Spaces around binary Ops

Preference for operator is ?. (QUESTION_DOT), Operation is analogous to DOT except that a nil ref short ccts and returns nil for the expression (chain).

# raw: name is nil or trimmed capitalized string
name = customer?.name?.trim.capitalized 

# w/nilCoalesce: name is empty string or trimmed capitalized string
name = customer?.name?.trim.capitalized ? ""

Should handle non idempotent calls/derefs.

If ambiguity between this and nil coalese op on trailing member ref then need space wrapped ops which should be done (if done) as a separate step.

[hopscc]

Code changes and tests for nilsafe deref (using '?.' as operator)
Always captures derefs to internal temp var for non idempotent call handling

No attention given to ambiguity to nil-coalesce with dot ref following

result = a.b ? .other # nil coalesce test
# treated as 
result = a.b?.other # nilsafe deref  to .other
# and probably fail

can correct using a tmp variable

tmpOther = .other
result = a.b ? tmpOther

[Charles]

related to ticket:341 - "Require (some) binary operators to have spaces around them"

[hopscc]

rework patch as suggested in Forum discussion page 4.
Use .isImplicit and add more commentary on what is being done with AST nodes synthesized for nilsafe deref op.
Comment tests as to use of inverse coalesce operator sequences vs nilSafe deref op.