Rvm optimizations (microsoft#620)

* perf(rvm): fix O(n²) comprehension yield by mutating in-place

Instead of cloning the entire accumulator collection on every yield
iteration, use take_register + Rc::make_mut to get exclusive ownership
and mutate in-place. This reduces comprehension yield from O(n²) to O(n)
for both run-to-completion and suspendable execution modes.

- Add RegoVM::take_register() helper that swaps register with Undefined
- Comprehension yield now takes the accumulator, mutates via Rc::make_mut,
  and writes back — avoiding deep clones when refcount == 1

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

* perf(rvm): use take_register for ObjectSet, ArrayPush, SetAdd

These instructions were cloning the container register (bumping Rc to 2),
then calling as_object_mut/as_array_mut/as_set_mut which invokes
Rc::make_mut — deep-cloning the entire collection since refcount > 1.

Use take_register instead so the Rc refcount stays at 1, making
Rc::make_mut a no-op and allowing in-place mutation.

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

* perf(rvm): remove unnecessary clones in rule caching

- execute_call_rule_common: move final_value into cache instead of
  cloning, since it is not used afterwards
- finalize_rule_frame_data: add comment clarifying the clone is needed
  because the value is both cached and returned
- Remove unnecessary .clone() on result_from_rule when setting register

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

* rvm: avoid RuleInfo clone per rule call

Replace RuleInfo.clone() (which heap-allocates name, destructuring_blocks, and
potentially function_info) with a cheap Arc<Program> clone (atomic refcount
bump) followed by borrowing &RuleInfo from the local Arc. This eliminates
per-rule-call heap allocations.

Sites changed:
- execute_call_rule_common: Arc clone + borrow
- execute_call_rule_suspendable: Arc clone + borrow
- finalize_rule_frame_data: Arc clone + borrow
- handle_rule_break_event: inline Arc clone + borrow (was get_rule_info)
- handle_rule_error_event: inline Arc clone + borrow (was get_rule_info)
- Removed now-unused get_rule_info method

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

* rvm: replace bincode with postcard for serialization

Remove unlinked bincode dependency. Use postcard (already a dep for rvm feature)
for all binary serialization/deserialization in program serialization and tests.

Also adds rvm_benchmark benchmark.

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

* perf(rvm): cache dummy Span/Expr for builtin calls

Every builtin call was allocating a Source (via from_contents), a Span, and
N Ref<Expr> wrappers just to satisfy the builtin function signature. These
dummy values are only used for error reporting context.

Cache the dummy Span and Vec<Ref<Expr>> on the RegoVM struct. The Source and
Span are created once on first builtin call; dummy Expr entries grow as
needed and are reused across calls via mem::take/put-back pattern.

This eliminates per-builtin-call heap allocations for Source (Rc + String +
Vec<lines>), Span clones, and Rc<Expr> wrappers.

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

* perf(rvm): round 2 allocation reduction in builtins, entry points, virtual data

- Cache builtin args Vec on RegoVM (mem::take/clear/put-back pattern)
- Restructure builtins_cache as two-level map for clone-free lookup
- Use IndexMap::get_index() in execute_entry_point_by_index
- Use mutable Vec path stack in traverse_rule_tree_subobject (push/pop)
- Walk data tree and rule-result paths by reference, clone only leaf
- Use mem::replace in resume() instead of cloning ExecutionState

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

* fix(rvm): address PR review feedback

- Restore cached_builtin_args on all error/early-return paths in
  execute_builtin_call to preserve allocation reuse
- Use 1-based line/col and \"<builtin>\" filename in dummy span for
  clearer diagnostics
- Restore result register before returning errors in comprehension
  mode-mismatch branches (both run-to-completion and suspendable)
- Avoid clone in resume() invalid-state error path by formatting
  debug string before moving state back

---------

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

Author: anakrish

Cargo.toml

@@ -189,6 +189,11 @@ harness = false
 name = "aci_benchmark"
 harness = false
 
+[[bench]]
+name = "rvm_benchmark"
+harness = false
+required-features = ["rvm"]
+
 [[example]]
 name="regorus"
 harness=false