Improve Susan buffers

[sjoelund.se]

Susan has performance issues with large strings sometimes. I think adding a new language element and changing a few buffers could be a possible solution:

let &buf = buffer ""
to
let &buffer = fileBuffer "someFile" ""

[sjoelund.se]

Or perhaps (on an empty/non-empty Tpl.Text as input):

let () = redirectTplToFile("someFile")

Or:

redirectTplToFile("someFile") + "rest"

(Because I guess that would chain the Tpl.Text better in the current Susan generator.

[sjoelund.se]

It is feasible, but requires the user to rewrite some templates to consider streaming from top to bottom without temporary buffers and so on. Any let-expression will be calculated in a separate Tpl.Text (in-memory) and written later. This also means any temporary ticks, etc. are hard to write in an efficient manner (so we should get out new data structure for a new code generator as soon as possible while fulfilling all of my requirements for it).

[sjoelund.se]

Partially fixed by eadffd4