First proposal of consistency checks

[bkamins]

This fixes #1885 and #1845.

What it does:

• sanitizes push! and append!, as I concluded that it is best to do more strict checks there (as we do dynamic dispatch there anyway)
• adds consistency checks to join, groupby and _show

(this also lead to a bit different approach to error throwing)

Consistency check for 10000 column costs ~200 microseconds so I think it is acceptable. But please comment on this.

Also please comment where else you think it is worth to add consistency checks.

[nalimilan]

Looks good. Though that won't make append! catch problems similar to #1885 but when appending a third data frame: in that case we have no idea that x2 has been corrupt until show is called. But at least that makes the error more visible.