Replace SectionBuilder with function builders

[litso]

Adds function builder support to Section and Sections:

let sections = Sections {
    Section(name: "a section") {
        Row(value: "a row")
        Row(value: "another row")
    }
    Section(name: "another section") {
        Row(value: "another other row")
        if someCondition {
            Row(value: "a conditional row")
        }
    }
}

This replaces the existing SectionBuilder, and also raises the iOS deployment target and Swift version, so this is a breaking change.