SI-5513: add inplace set-theoretic operations for mutable bitsets.
Warn about forgotten string interpolators.

In the compiler sources this arrives with a number of false
positives, because we frequently work with strings containing
$foo where foo is an in-scope identifier. I think in normal
source code this will be less of a problem, or none at all;
but to be conservative the warning is born under -Xlint.
Eliminate a bunch of -Xlint warnings.

Mostly unused private code, unused imports, and points where
an extra pair of parentheses is necessary for scalac to have
confidence in our intentions.
Removed dead src directory.

One line of commentary referenced from nowhere can't be very useful.
Optimize rebalance method by using null optimized list implementation.

rebalance method relies heavily on s.c.i.List. By replacing List with null
optimized version, NList, rebalance operation is significantly more faster.
Test indicate +10 % performance improvement for tree sizes >= 100.

Existing tests verify red-black tree invariants, including operations
involving tree rebalance.
SI-7236 Deprecate ThreadPoolTaskSupport and friends
removed a redundant var in JavaWriter.flagsToStr

the variable x was assigned to, but was never used.
Merge pull request #2219 from chuvoks/rbtRebalance

Optimize RedBlackTree rebalance method by using null optimized list implementation.
Merge pull request #2235 from vigdorchik/si_5513

SI-5513: add inplace set-theoretic operations for mutable bitsets.
Merge pull request #2251 from soc/SI-7236

SI-7236 Deprecate ThreadPoolTaskSupport and friends
Merge pull request #2262 from mesagie/patch-4

removed a redundant var in JavaWriter.flagsToStr
Merge pull request #2239 from paulp/pr/warnings

Warnings removal and other cleanup.