#167 feedback, but as a PR

[michaelglass]

two small commits on top of #167:

1. Some col is a column everywhere, so no stripSome

this where threw on #167:

where (SqlFn.nullif (SqlFn.upper a.city, "") = Some a.addressline1)
// before (#167): NotImplementedException "Unable to evaluate query parameter expression: Some(a.addressline1)"
// after:         WHERE (NULLIF(UPPER(a.city), '') = a.addressline1)

same root cause as the on' case #167 fixed with stripSome: NProperty already treats Some col as a column, but some SQL-function arm runs NValue first, and NValue tries to evaluate Some(col). so the fix moves into NValue: it now says "not a value" for Some of a query column.

before (#167, join predicate only):

let stripSome (nexp: NormalizedExpression) =
    match nexp with
    | NMethodCall (m, [ (NColumn _) as inner ]) when m.Method.Name = "Some" -> inner
    | _ -> nexp
// ...
let left, right = stripSome left, stripSome right

after (in NValue, shared by where / having / joins):

| NMethodCall(call, [ arg ]) when call.Method.Name = "Some" && isQueryVariable arg -> None

your #167 on' tests still pass without stripSome, and the where case above is a new regression test (fails on #167, passes here).

2. one helper for the three join'/leftJoin' bodies

before, three copies of:

let innerAlias =
    match resultSelector.Parameters |> Seq.toList with
    | [_; inner] -> inner.Name
    | _ -> failwith "Expected two parameters in leftJoin result selector"
let _, innerTableMappings = TableMappings.tryGetByRootOrAlias innerAlias innerSource.TableMappings
let mergedTables = mergeTableMappings (outerSource.TableMappings, innerTableMappings)
let innerTable = mergedTables[TableAliasKey innerAlias]
let tableName = FQ.qualifiedTable innerTable
let pendingJoin = { JoinType = JoinType.Left; TableName = tableName; TableAlias = innerAlias }
let ir = mergeCtes (outerSource |> getQueryOrDefault) innerSource
this.PendingJoinInfo <- Some pendingJoin
QuerySource<'JoinResult, SelectQueryIR>(ir, mergedTables)

after:

let pending, mergedTables = pendingJoin JoinType.Left resultSelector.Parameters outerSource.TableMappings innerSource.TableMappings
this.PendingJoinInfo <- Some pending
QuerySource<'JoinResult, SelectQueryIR>(mergeCtes (outerSource |> getQueryOrDefault) innerSource, mergedTables)

also added a test that a cteFrom on the inner side of join'/leftJoin' keeps its WITH. nothing covered that before.