After the previous commit, nushell uses PrettyDebug and
PrettyDebugWithSource for our pretty-printed display output.
PrettyDebug produces a structured `pretty.rs` document rather than
writing directly into a fmt::Formatter, and types that implement
`PrettyDebug` have a convenience `display` method that produces a string
(to be used in situations where `Display` is needed for compatibility
with other traits, or where simple rendering is appropriate).
This commit extracts Tag, Span, Text, as well as source-related debug
facilities into a new crate called nu_source.
This change is much bigger than one might have expected because the
previous code relied heavily on implementing inherent methods on
`Tagged<T>` and `Spanned<T>`, which is no longer possible.
As a result, this change creates more concrete types instead of using
`Tagged<T>`. One notable example: Tagged<Value> became Value, and Value
became UntaggedValue.
This change clarifies the intent of the code in many places, but it does
make it a big change.
fixes#969, admittedly without a --delimiter alias
moves from_structured_data.rs to from_delimited_data.rs to better
identify its scope and adds to_delimited_data.rs. Now csv and tsv both
use the same code, tsv passes in a fixed '\t' argument where csv passes
in the value of --separator
`save` attempts to convert input based on the target filename extension,
and expects a stream of text otherwise. However the error message is
unclear and provides little guidance, hopefully this is less confusing
to new users.
It might be worthwhile to also add a hint about adding an extension,
though I'm not sure if it's possible to emit multiple diagnostics.
