add stictness to vector updating in histogram_ to avoid space leak

[alexandersgreen]

When using the Kernel Density Estimation algorithm, we detected a space leak that causes a stack overflow for large inputs. After some profiling, this was discovered to be due to a lack of strictness in the histogram_ function when it updates an element in a vector.
This change simply uses a bang pattern to introduce strictness in that function as necessary to remove the space leak, and enable us to run the Kernel Density Estimation functions in constant space.

[bos]

Thanks!