In the long run collections is the right thing to use, but until we can
require 3.9.x as the minimum we cannot annotate the collections.abc
version, only the typing version. This allows correct type annotations
in users of `CompilerArgs`, as mypy (and friends) can determine that
`CompilerArgs` is ~= `T.Sequence[str]`
pre_flush_set and post_flush_set are almost always empty, so we can use
extend() instead of a for...in loop to add the previous elements of
self._container.
We can also skip the conversion from deque to list since pre_flush is
always appended on the right side.
On a QEMU build the time spent in flush_pre_post goes from 1.4 to 0.5
seconds.
Since the CompileArgs class already needs to know about the compiler,
and we really need at least per-lanaguage if not per-compiler
CompilerArgs classes, let's get the CompilerArgs instance from the
compiler using a method.
Josh Soref