* 1. Split ColumnOp into ColumnOp & FieldOp, former storing ColId
2. Shrink SqlAst to 80 bytes, so it can be passed in registers
3. Store end-result Header in IndexSemiJoin
4. Remove operational use of Header in ColumnOp & build_query
5. Simplify RowRef::{get, project, project_owned}
* Make parts of build_query actually infallible.
1. Make IndexSemiJoin::filter infallible.
2. Make ColumnOp::compare and friends infallible.
3. Make RowRef::{get, project, project_owned} infallible.
* Make RelOps::next itself infallible
* 1. with_select{_cmp}: ensure type safety o query exec cannot panic
2. Document RelValue::{get, read_or_take_column, project_owned}
3. Refactor optimize_select
4. Ensure in optimize_select that conditions are merged with preceding selects
* remove RelOps::{head, row_count}; head is redundant & row_count is useless
* remove Relation trait; it does not carry its weight
* make build_query infallible
* simplify IndexSemiJoin, make it slightly less branchy
* simplify try_index_join
* split IndexSemiJoin into Left & Right parts
* move get_field_pos to test code
* move test version of build_query to test code
Viktor Szépe