[flang] Initial buffer framing code

Address review comments

Integer output data editing (I,B,O,Z)

Full integer output formatting

Stub out some work in progress

Progress on E output data editing

E, D, EN, and ES output editing done

Fw.d output editing

Real G output editing

G output editing for reals

Make environment a distinct module

CHARACTER and LOGICAL output editing

Minimal decimal representations for E0, F0, G0 editing

Move real output editing code into its own file

Fix/dodge some GCC build problems

Prep work for external I/O statement state

External HELLO, WORLD

Fix build problem with GCC

Add virtual destructors where needed

Add new test

Original-commit: flang-compiler/f18@c3f1774f8eee903928b7e46636edfb03425eabc0
Reviewed-on: https://github.com/flang-compiler/f18/pull/950