[Jonathan-Schaefer-git]

Fixes #128.

The F# kernel checks a cell for diagnostics, hover and completions in one virtual script that holds every executed cell followed by the cell being checked. Once a cell that defines a type or a module has run, its definitions appear twice in that file, and checking it again reports FS0037 on each of them.

FSI compiles each execution as a fragment of its own and opens it for the ones after. The virtual script imitates this.

Each executed cell is its own module. Each executed source is wrapped in a module of its own, followed by an open of that module. The cell being checked stays at the top level, so its positions are unchanged.
Directives still apply to the whole script. #r, #load, #I, #nowarn and the other script directives are lifted out of the module, since they apply to the whole script.
Opens stay open. A fragment’s top-level opens are repeated after it, since in FSI they stay open for later cells.
The wrappers stay invisible. Completions leave the wrapper modules out, and hover text and diagnostic messages drop their names, so a type reads as Notebook.Shape rather than Notebook.VersoExecutedCell1.Shape.

ExecutedCellsTests covers the cases:

a run cell checked again has no duplicate definitions;
a value redefined by a later cell takes the later definition;
opens persist, including the open of a module from the same cell, and a module from a run cell can be opened later;
an #r reference still resolves in later cells;
a cell with a triple-quoted string still checks;
the wrapper modules stay out of completions, hover and diagnostics.

[TorreyBetts]

Thanks for this, looks good. Will be included in the next release.