New collect operator

[pditommaso]

Introduce a new collect operator that allow to collect the items produced by a channel into list object.

The collect operator has the following advantages over the existing toList object.

• Uniform name with existing Collect#collect Groovy method.
• Add the ability to specify a closure to transform the item before adding it to the result list.
• Emit a stop signal - instead of a empty list - when applied to an empty channel. Fix Improve to toList operator to return an empty channel instead of an empty list #131.
• Order of collected items do not invalid process cache. Fix Operator toList invalidates process cache #207.
• List objects are automatically merged into the resulting list.

Examples:

Channel
    .from( 1, 2, 3, 4 )
    .collect()
    .println()

# outputs
[1,2,3,4]

The following example shows how use a mapper closure:

Channel
    .from( 'hello', 'ciao', 'bonjour' )
    .collect { it.length() }
    .println()

# outputs
[5,4,7]

[pditommaso]

Included in version 0.24.0