chore(data/matrix/basic): rename _val lemmas to _apply (#3297)
We use `_apply` for lemmas about applications of functions to arguments. Arguably "picking out the entries of a matrix" could warrant a different name, but as we treat matrices just as functions all over, I think it's better to use the usual names.
Co-authored-by: Scott Morrison <scott.morrison@gmail.com>
Co-authored-by: Bryan Gin-ge Chen <bryangingechen@gmail.com>
