Add support for slice

[kriskowal]

No description provided.

[kriskowal]

@aadsm, would you mind taking on the reivew+merge/maybe?

[aadsm]

What are the nuances / caveats on array.slice?

[kriskowal]

@aadsm,

 +The behavior for strings is unimaginative.
 +If the source string, or either the start index or end index changes, the target
 +will be replaced with that slice of the source.
 +
 +The behavior for arrays is nuanced.
 +To avoid these nuances, you should generally use the `view(start, length)`
 +operator directly.
 +It is better suited for viewing a sliding window of an array, allowing
 +the position and size of the window to change orthogonally.
 +
 +However, if you happen to use the `slice` operator on an array, you can expect
 +it to deviate from the norm.
 +Most FRB operators that produce arrays will only emit that array once, even if
 +the value of the input changes.
 +This is beneficial because one can attach change listeners once to an output
 +array knowing that these change listeners will continue to react until the
 +binding is canceled, even if the input temporarily null or is replaced.
 +Because the `slice` operator may have either a string or an array as its input,
 +the output value will change if ever the input value is replaced and may switch
 +between being an array or string depending on the input type.