Merge #580: Add trivial ecmult_multi algorithm which does not require a scratch space

Merge #580: Add trivial ecmult_multi algorithm which does not require a scratch space

a697d82 Add trivial ecmult_multi to the benchmark tool (Jonas Nick)
bade617 Add trivial ecmult_multi algorithm. It is selected when no scratch space is given and just multiplies and adds the points. (Jonas Nick)

Pull request description:

  This commit adds a new ecmult_multi algorithm that is automatically selected when `ecmult_multi_var` is called with scratch space set to `NULL`. This is a trivial algorithm that simply multiplies the points with the corresponding scalars and adds them up.

  The use case is to allow creating exposed function that uses `ecmult_multi` but without requiring a scratch space argument. For example, in MuSig when computing the combined public key we need to compute a weighted sum of points but we most likely don't care about the performance. And if we do we can still provide a scratch space. Having the option of not providing a scratch space is useful because creating a scratch space is not entirely trivial. One needs to decide on a size and it needs to be destroyed.

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