Pattern matching in "do" requires a MonadFail context, which we don't have in
pure code. Instead, we'll use "case-of" to bind the part of the state that
we're interested in.

It used to only treat all trailing variables in read as varaible
declarings, but an array variable can be declared in other positions:

    read -a foo -r

foo is a declared variable, and multiple such variables can be declared.

Signed-off-by: NMartin Schwenke <martin@meltin.net>

Specifies the maximum severity of errors to handle.  For example,
specifying "-S warning" means that errors of severity "info" and
"style" are ignored.
Signed-off-by: NMartin Schwenke <martin@meltin.net>

Fixes #1310

'@' was previously mentioned in 5005dc0f as a
character needed to fix #909, but was not included
in the actual change at that time.

Fixes #1268