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-command/src/strings/format/date.rs

@@ -159,7 +159,7 @@ fn format_helper(value: Value, formatter: &str, formatter_span: Span, head_span:
         }
         _ => Value::error(
             ShellError::DatetimeParseError {
-                msg: value.debug_value(),
+                msg: value.to_debug_string(),
                 span: head_span,
             },
             head_span,
@@ -180,7 +180,7 @@ fn format_helper_rfc2822(value: Value, span: Span) -> Value {
         }
         _ => Value::error(
             ShellError::DatetimeParseError {
-                msg: value.debug_value(),
+                msg: value.to_debug_string(),
                 span,
             },
             span,

crates/nu-command/src/strings/format/duration.rs

@@ -67,7 +67,7 @@ impl Command for FormatDuration {
     ) -> Result<PipelineData, ShellError> {
         let format_value = call
             .req::<Value>(engine_state, stack, 0)?
-            .as_string()?
+            .coerce_into_string()?
             .to_ascii_lowercase();
         let cell_paths: Vec<CellPath> = call.rest(engine_state, stack, 1)?;
         let cell_paths = (!cell_paths.is_empty()).then_some(cell_paths);

crates/nu-command/src/strings/format/filesize.rs

@@ -64,7 +64,7 @@ impl Command for FormatFilesize {
     ) -> Result<PipelineData, ShellError> {
         let format_value = call
             .req::<Value>(engine_state, stack, 0)?
-            .as_string()?
+            .coerce_into_string()?
             .to_ascii_lowercase();
         let cell_paths: Vec<CellPath> = call.rest(engine_state, stack, 1)?;
         let cell_paths = (!cell_paths.is_empty()).then_some(cell_paths);