Add Base.copy(::Node)

[TimG1964]

I've added a copy function and some associated tests. It recursively copies a node and all its children. A little discussion on Discourse, here.

I've added some tests for this, too.

The Readme page says:

• Node is an immutable type. However, you can easily create a copy with one or more field values changed by using the Node(::Node; kw...) constructor where kw are the fields you want to change. For example:

node = XML.Element("tag", XML.Text("child"))

simple_value(node)
# "child"

node2 = Node(node, children=XML.Text("changed"))

simple_value(node2)
# "changed"

I think this is probably out of date. The Node() function now seems to treat arguments as (new) children and keywords as new/changed attributes. I've changed the text here to reflect this and added a reference to copy().

[TimG1964]

Something strange here. There are old changes in the code I thought I'd removed when I synced my fork just now. Evidently not! I'll close this and try harder!