Implement proper column versions

[karoliskoncevicius]

Currently the col_ versions of all tests transpose the inputs and call row_ functions.

This behaviour is undesirable because of at least three reasons:

1. Transposing inputs takes time.
2. col-wise functions (like colMeans) are typically faster than their row equivalents
3. When inputs have different lengths the functions might throw a warning about rows.

• row_t_onesample(x)
• row_t_welch(x, y)
• row_t_equalvar(x, y)
• row_t_paired(x, y)
• row_wilcoxon_onesample(x)
• row_wilcoxon_twosample(x, y)
• row_wilcoxon_paired(x, y)
• row_cor_pearson(x, y)
• row_oneway_welch(x, g)
• row_oneway_equalvar(x, g)
• row_kruskalwallis(x, g)
• row_f_var(x, y)
• row_bartlett(x, g)
• row_flignerkilleen(x, g)
• row_jarquebera(x)