Begin migration away from arg serialization (#3281)

* initial implementation

* Move a few commands over to new arg system

* Fix char also

crates/nu-protocol/src/value.rs

@@ -429,6 +429,14 @@ impl Value {
         matches!(&self.value, UntaggedValue::Primitive(_))
     }
 
+    /// View the Value as unsigned size, if possible
+    pub fn as_usize(&self) -> Result<usize, ShellError> {
+        match &self.value {
+            UntaggedValue::Primitive(primitive) => primitive.as_usize(self.tag.span),
+            _ => Err(ShellError::type_error("integer", self.spanned_type_name())),
+        }
+    }
+
     /// View the Value as unsigned 64-bit, if possible
     pub fn as_u64(&self) -> Result<u64, ShellError> {
         match &self.value {

crates/nu-protocol/src/value/primitive.rs

@@ -60,6 +60,30 @@ pub enum Primitive {
 }
 
 impl Primitive {
+    /// Converts a primitive value to a u64, if possible. Uses a span to build an error if the conversion isn't possible.
+    pub fn as_usize(&self, span: Span) -> Result<usize, ShellError> {
+        match self {
+            Primitive::Int(int) => int.to_usize().ok_or_else(|| {
+                ShellError::range_error(
+                    ExpectedRange::U64,
+                    &format!("{}", int).spanned(span),
+                    "converting an integer into an unsigned 64-bit integer",
+                )
+            }),
+            Primitive::Decimal(decimal) => decimal.to_usize().ok_or_else(|| {
+                ShellError::range_error(
+                    ExpectedRange::U64,
+                    &format!("{}", decimal).spanned(span),
+                    "converting a decimal into an unsigned 64-bit integer",
+                )
+            }),
+            other => Err(ShellError::type_error(
+                "number",
+                other.type_name().spanned(span),
+            )),
+        }
+    }
+
     /// Converts a primitive value to a u64, if possible. Uses a span to build an error if the conversion isn't possible.
     pub fn as_u64(&self, span: Span) -> Result<u64, ShellError> {
         match self {