Return the number of elements in an rdeque
Returns the number of elements in an rdeque.
## S3 method for class 'rdeque' length(x)
rdeque to get the length of.
Runs in O(1) time, as this information is stored seperately and updated on every insert/remove.
a vector of length 1 with the number of elements.
empty for checking whether an rdeque is empty.
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