Tom Almeida authored Mar 17, 2020 and AdminXVII committed Apr 20, 2020

Before this patch, `let` only shows string and array variables, whilst
all mapping types are not shown. In addition, string and array variables
are separated from each other by comments.

This patch implements a new function on `shell::Variables` that returns
all the variables in scope and then modifies `Shell::list_vars` to print
all the variables. The syntax returned is equivalent to the method of
declaring the variables themselves, however this still results in
mapping types being indistinguishable from each other by looking at the
output of `let` (both `HashMap` and `BTreeMap` look the same).