With this change, the following is now valid syntax:

let array = [
    one two
    three # a comment
    four # another comment
]

echo $join(array, "\n")

Since we are depending on Nightly, we may as well utilize the impl
trait return type feature that should help us to avoid some heap
allocations when getting a list of variables in the shell.

After the refactoring of how let and other builtins were parsed, we
lost the ability to get a list of local variables just by supplying
the let command without any arguments. This will re-implement that
ability.

Liner seems to be doing some very limited form of multi-line comment
support, itself. This change will strip out the excess characters
that are returned by liner.

Update variables for commands that start with ~, too
Closes #551

A bit of refactoring to make handling method arguments simpler.

Lukáš Král authored 7 years ago and Michael Aaron Murphy committed 7 years ago

* Fixed is_array to return false on "[1 2 3][0]"

* Added derive for Copy to IsArrayHelper

Closes #549

Addresses precision issues with floating point math by converting
all usage of f64 to d128 from the decimal crate.

Closes #529

Closes #544

Closes #543

Fixes issues like the following:
    ~/.cargo/bin/ion -c echo test
    let COM = echo; $COM test

To use a function to generate a prompt, simply create a function whose name is PROMPT.