Reduce memory allocation when increasing the number of coefficients for SimHash

[kernelmethod]

Currently, in order to sample new coefficients for SimHash, we use the following code:

@views new_coeff[old_n+1:end,1:end] = randn(n - old_n, n_hashes)

It'd be more efficient to run map! on the new_coeff array, since the code above allocates a new array of size (n-old_n) x n_hashes which is immediately thrown away.