add -Xsource:version to scalac man page

The flag was added in d43618a (PR #3340) by @huitseeker.
SI-8266 Deprecate octal escapes in f-interpolator

Also turns the f-interpolator into a migration
assistant by suggesting alternatives for the
standard escapes.
SI-6908 FlatHashTable and things that depend on it can't store nulls

Fixed ParFlatHashTable to use entryToElem which correctly converts sentinels to nulls.
SI-7711 Do not emit extra argv in script body

Take away `argv` and make `args` the standard parameter name.

This is a quick fix to avoid "unused local" lint error.  All
the examples use `args`; in particular, "Step 4. Write some
Scala scripts" in "Programming in Scala" uses `args`.

I see the footnote there is also where Odersky concatenation is
specified, `"Hello, "+ args(0) +"!"` with no space next to the
literals.

Also removes `argv` from `StdNames`. Was torn whether just to
add `argc`. Maybe start a new project to house Names, emeritus.
SI-7711 Test for args in script

Compiles with -Xlint to ensure there are no lurkers.

Updates `ScriptTest` to pass args to the script.

It's called `argv` partly as homage, partly because
`args` is an `Array`.
Add an extremely well-commented test

This commit includes a test for some simple existential subtyping
checks. It is exceptionally well-commented and may be helpful to
someone trying to figure out what the rules are (supposed to be)
in the future.
SI-8264 scala.collection.immutable.HashSet#- returns broken Set

Was an error in HashSet's handling of removal of an element when a HashTrieSet should turn into a HashSet1.

Also slightly modified HashMap's filter0 to more closely match HashSet (by adding the same comment).
Made ParFlatHashTableIterator private vars private[this].
Merge pull request #3534 from clhodapp/test/literate_existentials

Add an extremely well-commented test
Merge pull request #3532 from som-snytt/issue/7711

SI-7711 Do not emit extra argv in script body
Merge pull request #3443 from gourlaysama/man-pages

add -Xsource:version to scalac man page
Merge pull request #3527 from Ichoran/issue/8264

SI-8264 scala.collection.immutable.HashSet#- returns broken Set
Merge pull request #3528 from Ichoran/issue/6908

SI-6908 FlatHashTable and things that depend on it can't store nulls
Merge pull request #3511 from som-snytt/issue/interp-octal

SI-8266 Deprecate octal escapes in f-interpolator