Revert "Stat: modularize as FileStat module."

This reverts commit 97f1a191b78a15ed6a876d5d56d93c9b468be43d.

See GitHub discussion.
print_with_color(msg, color) => print_with_color(color, msg...)

Change print_with_color, info and warn to allow many message pieces.
JL_ANSWER_COLOR => JULIA_ANSWER_COLOR; JL_POST_BOOT gone [close #2076]

JL_PRIVATE_LIBDIR remains and I wasn't sure what to do about that. It
seems to be unused at this point -- or at least I couldn't find any
usages of it.

This commit also changes the way one controls the answer color during
a repl session: there is now a Main.ANSWER_COLOR variable that you can
modify to change the answer color on the fly. The environment variable
JULIA_ANSWER_COLOR is only checked upon repl startup and not after.
Update docs to reflect recent deprecations
Stat: modularize as FileStat module.
Improve documentation of function signatures