refactor: factor out text handling as new modules Text and Loctext

[favonia]

The current Diagnostic module bundles diagnostics and text operations together. The planned lower-level debugging interface #138 could reuse text operations but not (high-level) diagnostics. This PR factors out text operations as new modules Text and Loctext so that the debugging interface does not have to depend on the Diagnostic module.

Here are all the bindings being moved:

Old binding	New binding
Diagnostic.text (type)	Text.t
Diagnostic.text (function)	Text.make
Diagnostic.textf	Text.makef
Diagnostic.ktextf	Text.kmakef
Diagnostic.loctext (type)	Loctext.t
Diagnostic.loctext (function)	Loctext.make
Diagnostic.loctextf	Loctext.makef
Diagnostic.kloctextf	Loctext.kmakef
Diagnostic.string_of_text	Text.to_string

[mikeshulman]

Seems reasonable, looking in from the outside.