Row scopes collection, breadth-first scope search

[groue]

This PR enhances row scopes. The changes only impact hard-core users, GRDB internals, and pave the way to associations in GRDB3.

Row scopes are now accessed through a dedicated collection:

// Define a tree of nested scopes
let adapter = ScopeAdapter([
    "foo": RangeRowAdapter(0..<1),
    "bar": RangeRowAdapter(1..<2).addingScopes([
        "baz" : RangeRowAdapter(2..<3)])])

// Fetch
let sql = "SELECT 1 AS foo, 2 AS bar, 3 AS baz"
let row = try Row.fetchOne(db, sql, adapter: adapter)!

row               // [foo:1, bar:2, baz:3]

row.scopes.count  // 2
row.scopes.names  // ["foo", "bar"]

row.scopes["foo"] // [foo:1]
row.scopes["bar"] // [bar:2]
row.scopes["baz"] // nil

for (name, scopedRow) in row.scopes {
    print(name, scopedRow)
}

The tree of nested scopes has its own dedicated view as well, that performs a breadth-first search of scope names:

row.scopesTree.names  // ["foo", "bar", "baz"]

row.scopesTree["foo"] // [foo:1]
row.scopesTree["bar"] // [bar:2]
row.scopesTree["baz"] // [baz:3]

This breadth-first search in the tree of nested scopes is now used when extracting records from rows. This will be paramount for associations ergonomics, so that the user can decode flat records from hierarchical scopes:

// From this hierarchical request made from associations...
let request = Book
    .including(required: Book.author
        .including(optional: Author.country))
    .including(optional: Bool.coverImage)

// ... you will decode flat records like the one below:
struct BookInfo: FetchableRecord, Decodable {
    var book: Book
    var author: Author
    var country: Country?
    var coverImage: CoverImage?
}
let bookInfos = try BookInfo.fetchAll(db, request) // [BookInfo]

// ... or perform explicit row decoding:
let rows = try Row.fetchCursor(db, request)
while let row = try rows.next() {
    let book = Book(row: row)
    let author: Author = row["author"]
    let country: Country? = row["country"]
    let coverImage: CoverImage? = row["coverImage"]
}