`char_mat`, `num_mat`, `trim_list`, `trim_vec` -- convert to character at the start

[Ananda Mahto]

Shouldn't add too much processing time, but would allow char_mat and num_mat to directly work on data.frames (as they are lists anyway).

Expect to work (after modification):

df <- data.frame(V1 = c("A", "B", "C"),
                 V2 = c("C", "C", NA),
                 V3 = c("D", "A", "B"))
char_mat(df)

This will presently generate an error.

[Ananda Mahto]

Taken care of in aeac3f7.