minor error message fix, minor internal tweaks

upsert with support for hardcoded conflict targets

allow disabling arg validation; internal improvements in errors

add "void" expressions without `returning *`

rename `.Append` to `.AppendTo`

add `LaxDict`

Additional cosmetic change: avoid single-letter variable names.

fix edge case bug in `Upsert`

fixed support for non-empty sparse structs in `Upsert`

add `Upsert`

`OrdsParser` supports default "dir" and "nulls" in tags