Function emit_array_alloc

pub(crate) fn emit_array_alloc(
    body: &mut InstrSeqBuilder<'_>,
    runtime: &RuntimeFunctions,
    source_pos: Option<MirSourcePos>,
    fallback_line: i32,
    memory_id: MemoryId,
    len_local: LocalId,
    dst_local: LocalId,
    scratch_i32_local: LocalId,
) -> Result<(), LowerError>

Allocate heap bytes and write array header with len/cap.