SearchTerm: add function to convert to bytestring

[frasertweedale]

We have to convert the SearchTerm to a C string to pass to libnotmuch.
Converting it to String then marshalling to CString is undesirable.

Ideally should use Builder to build the bytestring, convert to lazy bytestring, then convert
to strict bytestring. If we null terminate the string we can use Data.ByteString.Unsafe.unsafeUseAsCString to avoid the final copy that would be
incurred by B.useAsCString.

The query_create function should be updated to use B.ByteString at the same time.