Name the Value conversion functions more clearly (#11851)

# Description
This PR renames the conversion functions on `Value` to be more consistent.
It follows the Rust [API guidelines](https://rust-lang.github.io/api-guidelines/naming.html#ad-hoc-conversions-follow-as_-to_-into_-conventions-c-conv) for ad-hoc conversions.
The conversion functions on `Value` now come in a few forms:
- `coerce_{type}` takes a `&Value` and attempts to convert the value to
`type` (e.g., `i64` are converted to `f64`). This is the old behavior of
some of the `as_{type}` functions -- these functions have simply been
renamed to better reflect what they do.
- The new `as_{type}` functions take a `&Value` and returns an `Ok`
result only if the value is of `type` (no conversion is attempted). The
returned value will be borrowed if `type` is non-`Copy`, otherwise an
owned value is returned.
- `into_{type}` exists for non-`Copy` types, but otherwise does not
attempt conversion just like `as_type`. It takes an owned `Value` and
always returns an owned result.
- `coerce_into_{type}` has the same relationship with `coerce_{type}` as
`into_{type}` does with `as_{type}`.
- `to_{kind}_string`: conversion to different string formats (debug,
abbreviated, etc.). Only two of the old string conversion functions were
removed, the rest have been renamed only.
- `to_{type}`: other conversion functions. Currently, only `to_path`
exists. (And `to_string` through `Display`.)

This table summaries the above:
| Form | Cost | Input Ownership | Output Ownership | Converts `Value`
case/`type` |
| ---------------------------- | ----- | --------------- |
---------------- | -------- |
| `as_{type}` | Cheap | Borrowed | Borrowed/Owned | No |
| `into_{type}` | Cheap | Owned | Owned | No |
| `coerce_{type}` | Cheap | Borrowed | Borrowed/Owned | Yes |
| `coerce_into_{type}` | Cheap | Owned | Owned | Yes |
| `to_{kind}_string` | Expensive | Borrowed | Owned | Yes |
| `to_{type}` | Expensive | Borrowed | Owned | Yes |

# User-Facing Changes
Breaking API change for `Value` in `nu-protocol` which is exposed as
part of the plugin API.

crates/nu-cmd-dataframe/src/dataframe/values/nu_dataframe/conversion.rs

@@ -342,7 +342,7 @@ fn typed_column_to_series(name: &str, column: TypedColumn) -> Result<Series, She
             }
             DataType::String => {
                 let series_values: Result<Vec<_>, _> =
-                    column.values.iter().map(|v| v.as_string()).collect();
+                    column.values.iter().map(|v| v.coerce_string()).collect();
                 Ok(Series::new(name, series_values?))
             }
             DataType::Object(_, _) => value_to_series(name, &column.values),
@@ -561,7 +561,7 @@ fn input_type_list_to_series(
                 let value_list = v
                     .as_list()?
                     .iter()
-                    .map(|v| v.as_string())
+                    .map(|v| v.coerce_string())
                     .collect::<Result<Vec<String>, _>>()
                     .map_err(inconsistent_error)?;
                 builder.append_values_iter(value_list.iter().map(AsRef::as_ref));

crates/nu-cmd-dataframe/src/dataframe/values/nu_schema.rs

@@ -73,7 +73,7 @@ fn value_to_fields(value: &Value, span: Span) -> Result<Vec<Field>, ShellError>
                 Ok(Field::new(col, dtype))
             }
             _ => {
-                let dtype = str_to_dtype(&val.as_string()?, span)?;
+                let dtype = str_to_dtype(&val.coerce_string()?, span)?;
                 Ok(Field::new(col, dtype))
             }
         })