Overloaded interpolation in command literals

Depends on #134.

Command literals should support interpolation for strings and arrays.

Examples

1.

let words = ["hello", "world"] in `echo $words`

is equivalent to

`echo hello world`

When interpolating an array, each item of the array is an argument to the command.

2.

let words = ["hello", "new world"] in `echo $words`

is equivalent to

`echo hello "new world"`

When an array item contains spaces, that item is still treated as a single argument rather than multiple arguments.

3.

(\name -> `echo "hello $name"`) : String -> Cmd

When an expression is interpolated into a quoted argument, that expression is treated as a string.

4.

(\args -> `echo $args`) : ToArgs a => a -> Cmd

When an expression is interpolated and unquoted, that expression is treated as an argument list.

Implementation

[x] https://github.com/LightAndLight/ipso/issues/142

[x] Implement ToArgs class

class ToArgs a where
 toArgs : a -> Array String

instance ToArgs String where
 toArgs a = [a]

instance ToArgs a => ToArgs (Array a) where
 toArgs = flatMap toArgs

[x] Desugar interpolation inside commands into calls to toArgs
```
\a -> `echo $a`
```
elaborates to
```
\a -> cmd (["echo"] ++ toArgs a)
```
[x] Document ToArgs in reference

Notes

The use of flatMap means that nested arrays are flattened when used as command arguments. I'm okay with this, because I'm not sure there are any other useful ways to interpret nested arrays of command line arguments.

LightAndLight / ipso