Update varchar() to count bytes not chars

[lachlangh]

Fixes #960

This PR changes varchar() to measure the input length in bytes (nchar(x, type = "bytes")) rather than characters.
This matches SQL Server’s definition of VARCHAR(n) as the number of bytes, and prevents truncation when inserting multibyte UTF-8 strings.

Counting bytes instead of characters should not adversely affect other database backends.

Includes a minimal test verifying the multibyte case.

[detule]

Thanks again for your submission.

I see our varchar method gets used for NetezzaSQL as well. Off the top of my head I can't imagine your change impacting their workflow adversely so feels like fine to merge without doing extra work to check there.

LGTM.