2.0: Make NamedTuples not indexable by symbol

[jariji]

Generally, NamedTuples support the interfaces of both structs and tuples. But neither structs nor tuples support indexing by symbol. It may be convenient for situations with a non-identifier property name, but it mixes the semantics of getproperty and getindex confusingly by dispatching on the argument type.

julia> NamedTuple([Symbol("2") => 20, :a => 10])[Symbol("2")]
20

julia> NamedTuple([Symbol("2") => 20, :a => 10])[2]
10

getproperty should not decide its semantics based on the type of the argument --- for different behavior, use a different function. Imho, getindex(::NamedTuple, i) should index positions and getproperty should go by property name. These should be kept separate.

[adienes]

notably: would resolve basically all of these annoying inconsistencies #49932

[rafaqz]

The major drawback of this is that getindex has much better syntax for using NamedTuple for practical tasks when the key is a variable: my_nt[k] vs getproperty(my_nt, k). I agree with most of #49932 but there is no reason we cant change eachindex and keep getindex - Int also works in getindex.