WriterBase should allow writing to multiple locations in the string

[zspitz]

Currently, parameter declarations for the factory method formatter look like this:

Lambda(
    new [] {
        var s = Parameter(typeof(string), "s")
    }, 
    s
)

Ideally all the variable declarations should be either before the Lambda method call:

var s = Parameter(typeof(string), "s");

Lambda(
    ...

or perhaps after; not intermingled with the lambda call.

In order to do this, and still maintain the span for each expression tree node, we would have to either

1. allow moving the insertion point to somewhere else in the string builder, and then continually modify the spans from these other texts as more text is inserted at the original point, OR
2. allow creating another string builder, and then modifying the resulting span to be offset from the final results of the original string builder

[zspitz]

The only reasonable way to approach this is with 2.

We'd want to allow many string builders, not just two.

We'd need a Dictionary<string, StringBuilder>, perhaps with the empty string as the key for the main string builder.

How do we specify which string builder to write to? The safer way would be to pass the string builder name as an optional parameter to each writing method, Alternatively, we could have the concept of the active string builder; setting/unsetting some property could change the active string builder.

Let's try the safe way; there's not yet a public API for formatters.

[zspitz]

Conclusion:

• Dictionary<string, (StringBuilder, pathSpans)>
• WriteX methods take an optional string, for the stringbuilder name
• Once WriteNode has been called from the constructor, all string builder contents need to be merged into a single string builder. The order of the string builders is defined in the derived class. Each successive string builder will need its pathspans adjusted.
• ToString returns results from the final, merged, stringbuilder

[zspitz]

I don't think it possible to have the stringbuilder name as optional, so we'd have to change all the overloads of Write, WriteNode and WriteNodes.

Alternatively, we could have a SetActiveStringBuilder passing a string key to identify the current string builder. Each time this is called, it would have to be called again, to switch back to the "main" string builder.

NB We should replace the use of PreWrite with this mechanism.

[zspitz]

We've also replaced PreWrite, as noted above.