Don't close the stmt on bind error
Simplify bytes fetch by replacing nuodb_resultset_bytes with C.GoBytes

Downside: it's now allocating a separate byte slice for each bytes column
Replace most of the union casts with reinterpret_cast

Left only the int64_t <-> double conversions to avoid breaking
strict-aliasing rules.
Support empty column names better
Rename nuodb_ldflags.go to ldflags.go
Update readme regarding Go version and test instructions