feat!: add Rego Virtual Machine (RVM) implementation (#495)

* feat!: add Rego Virtual Machine (RVM) implementation

This commit introduces a register-based virtual machine for executing Rego
policies with bytecode-style instructions. Unlike the existing tree-walking
interpreter, the RVM compiles policies into instruction sequences that operate
on virtual registers, offering better performance and optimization potential.

Core Components:

Instruction Set Architecture:
- Define instruction types for data operations, control flow, and builtins
- Implement instruction parameter encoding and display formatting
- Add instruction parser with comprehensive test coverage

Virtual Machine Engine:
- Register-based execution model with program counter management
- Loop execution supporting iterators, comprehensions, and quantifiers
- Function call handling with argument evaluation and context management
- Rule evaluation with default value resolution and virtual data support
- Arithmetic and comparison operation implementations

Program Representation:
- Program listing builder with instruction sequencing
- Rule tree construction for organizing policy rules
- Binary and JSON serialization for compiled programs
- Recompilation support for program modification

Testing Infrastructure:
- Extensive YAML test suites covering all VM features
- Rust unit tests for VM execution and instruction parsing
- Test suites for loops, comprehensions, builtins, and control flow

BREAKING CHANGE: Introduces new VM execution path alongside interpreter

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

* docs: add detailed RVM architecture references

Introduce architecture.md explaining program artifacts, serialization, and runtime subsystems.
Document the full opcode catalog in instruction-set.md, including operands, parameter tables, and outcomes.
Walk through execution flow, stacks, and operational guidance in vm-runtime.md, tying the runtime to the new architecture docs.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

---------

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/value.rs

@@ -4,6 +4,7 @@
 use crate::number::Number;
 
 use alloc::collections::{BTreeMap, BTreeSet};
+use alloc::vec::Vec;
 use core::fmt;
 use core::ops;
 
@@ -67,7 +68,7 @@ impl Serialize for Value {
     {
         use serde::ser::Error;
         match self {
-            Value::Null => serializer.serialize_none(),
+            Value::Null => serializer.serialize_unit(),
             Value::Bool(b) => serializer.serialize_bool(*b),
             Value::String(s) => serializer.serialize_str(s.as_ref()),
             Value::Number(n) => n.serialize(serializer),
@@ -118,6 +119,20 @@ impl<'de> Visitor<'de> for ValueVisitor {
         Ok(Value::Bool(v))
     }
 
+    fn visit_none<E>(self) -> Result<Self::Value, E>
+    where
+        E: de::Error,
+    {
+        Ok(Value::Null)
+    }
+
+    fn visit_some<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+        Value::deserialize(deserializer)
+    }
+
     fn visit_u64<E>(self, v: u64) -> Result<Self::Value, E>
     where
         E: de::Error,