Make SpanId and RegId also use new ID struct (#13963)

# Description
In the PR #13832 I used some newtypes for the old IDs. `SpanId` and
`RegId` already used newtypes, to streamline the code, I made them into
the same style as the other marker-based IDs.

Since `RegId` should be a bit smaller (it uses a `u32` instead of
`usize`) according to @devyn, I made the `Id` type generic with `usize`
as the default inner value.

The question still stands how `Display` should be implemented if even.

# User-Facing Changes
Users of the internal values of `RegId` or `SpanId` have breaking
changes but who outside nushell itself even uses these?

# After Submitting
The IDs will be streamlined and all type-safe.

crates/nu-engine/src/compile/builder.rs

@@ -58,9 +58,9 @@ impl BlockBuilder {
                 }
             })
         {
-            Ok(RegId(index as u32))
+            Ok(RegId::new(index as u32))
         } else if self.register_allocation_state.len() < (u32::MAX as usize - 2) {
-            let reg_id = RegId(self.register_allocation_state.len() as u32);
+            let reg_id = RegId::new(self.register_allocation_state.len() as u32);
             self.register_allocation_state.push(true);
             Ok(reg_id)
         } else {
@@ -73,13 +73,16 @@ impl BlockBuilder {
     /// Check if a register is initialized with a value.
     pub(crate) fn is_allocated(&self, reg_id: RegId) -> bool {
         self.register_allocation_state
-            .get(reg_id.0 as usize)
+            .get(reg_id.get() as usize)
             .is_some_and(|state| *state)
     }
 
     /// Mark a register as initialized.
     pub(crate) fn mark_register(&mut self, reg_id: RegId) -> Result<(), CompileError> {
-        if let Some(is_allocated) = self.register_allocation_state.get_mut(reg_id.0 as usize) {
+        if let Some(is_allocated) = self
+            .register_allocation_state
+            .get_mut(reg_id.get() as usize)
+        {
             *is_allocated = true;
             Ok(())
         } else {
@@ -92,7 +95,7 @@ impl BlockBuilder {
     /// Mark a register as empty, so that it can be used again by something else.
     #[track_caller]
     pub(crate) fn free_register(&mut self, reg_id: RegId) -> Result<(), CompileError> {
-        let index = reg_id.0 as usize;
+        let index = reg_id.get() as usize;
 
         if self
             .register_allocation_state

crates/nu-engine/src/compile/mod.rs

@@ -18,7 +18,7 @@ use expression::compile_expression;
 use operator::*;
 use redirect::*;
 
-const BLOCK_INPUT: RegId = RegId(0);
+const BLOCK_INPUT: RegId = RegId::new(0);
 
 /// Compile Nushell pipeline abstract syntax tree (AST) to internal representation (IR) instructions
 /// for evaluation.

crates/nu-engine/src/eval_ir.rs

@@ -113,25 +113,28 @@ impl<'a> EvalContext<'a> {
     #[inline]
     fn put_reg(&mut self, reg_id: RegId, new_value: PipelineData) {
         // log::trace!("{reg_id} <- {new_value:?}");
-        self.registers[reg_id.0 as usize] = new_value;
+        self.registers[reg_id.get() as usize] = new_value;
     }
 
     /// Borrow the contents of a register.
     #[inline]
     fn borrow_reg(&self, reg_id: RegId) -> &PipelineData {
-        &self.registers[reg_id.0 as usize]
+        &self.registers[reg_id.get() as usize]
     }
 
     /// Replace the contents of a register with `Empty` and then return the value that it contained
     #[inline]
     fn take_reg(&mut self, reg_id: RegId) -> PipelineData {
         // log::trace!("<- {reg_id}");
-        std::mem::replace(&mut self.registers[reg_id.0 as usize], PipelineData::Empty)
+        std::mem::replace(
+            &mut self.registers[reg_id.get() as usize],
+            PipelineData::Empty,
+        )
     }
 
     /// Clone data from a register. Must be collected first.
     fn clone_reg(&mut self, reg_id: RegId, error_span: Span) -> Result<PipelineData, ShellError> {
-        match &self.registers[reg_id.0 as usize] {
+        match &self.registers[reg_id.get() as usize] {
             PipelineData::Empty => Ok(PipelineData::Empty),
             PipelineData::Value(val, meta) => Ok(PipelineData::Value(val.clone(), meta.clone())),
             _ => Err(ShellError::IrEvalError {