convenience constructors for Tables.Schema

[jrevels]

ref beacon-biosignals/Legolas.jl#54 (comment)

copy/pasting @ericphanson 's comment:

Random thought: if we are going to be using Tables.Schema a lot more (not sure if we are yet), it could be nice to upstream an Tables.@Schema(a::Int, b::Float64, c::String) that expands to Tables.Schema((:a, :b, :c), (Int, Float64, String)), maybe also a begin/end form like

Tables.@Schema begin
    a::Int
    b::Float64
    c::String
end

[quinnj]

Yeah, I'm down. Throw a PR up?