Language-integrated query

[jclark]

See issue #286 for a proposal that covers this.

Query can potentially apply to datatypes other than table, including lists, streams and XML.

[jclark]

For 2020R1, we are doing:

• Simple query expressions #398
• Allow spread operator in mapping constructors #399
• Variable name as field of mapping constructor #400
• Type inference for mapping constructor in select clause #401
• Generalize iterability to allow errors #402
• Add query-action #403
• Add stream basic type #406

[jclark]

To be done later in 2020R2:

• join clause (Add query clause for inner joins #435, Add query clause for outer joins #436)
• grouping and aggregation (Add query clauses for grouping and aggregation #441)
• order by clause (Add query clause for sorting #438)
• limit clause (Add query clause to limit number of results #437)
• query expressions that construct streams (Allow a query-expr to construct a stream #443)
• event streams (Support event streams #440)

[jclark]

2020R1 part now done.