wasm-encoder: add ComponentBuilder::instantiate_exports

[zacharywhitley]

The core-level analogue core_instantiate_exports already exists on
ComponentBuilder and mirrors the Instance::FromExports variant of
the core instance section. The component-level side had no equivalent:
ComponentInstanceSection::export_items is public but only reachable
through the manual section-append path, not the builder facade.

This adds instantiate_exports next to instantiate so callers
rebuilding a component structurally from a parsed one can round-trip
ComponentInstance::FromExports the same way core-level
Instance::FromExports already round-trips.

Signature

Mirrors core_instantiate_exports — same (&'a str, K, u32) triples,
just with ComponentExportKind in the kind slot instead of
ExportKind:

pub fn instantiate_exports<'a, E>(
    &mut self,
    debug_name: Option<&str>,
    exports: E,
) -> u32
where
    E: IntoIterator<Item = (&'a str, ComponentExportKind, u32)>,
    E::IntoIter: ExactSizeIterator,
{
    self.component_instances().export_items(exports);
    self.instances.add(debug_name)
}

Motivation

A component optimizer that reads a component with wasmparser and
re-emits it via ComponentBuilder needs to round-trip every
ComponentInstance variant. Instantiate was already covered by
ComponentBuilder::instantiate; FromExports was the missing case.
The public alternative was dropping down to
ComponentInstanceSection::export_items and manually appending the
section, bypassing the builder's index-space bookkeeping — which
defeats the point of using the builder.

Alternatives

• Expose the builder's internal component_instances() accessor:
  wider surface area than needed and doesn't match the pattern
  established by core_instantiate_exports.
• Require callers to use the raw section API: works but requires
  duplicating the builder's index-space accounting.

The chosen shape sticks to the precedent already set for the core
side.