Allow destructuring normal structs

[thautwarm]

While Match.jl provides such functionalities, in order to support users' redefinition of the way to destructure a struct, we disabled the default way to destructure normal structs like:

struct A
  a_1
  a_2
end

@match A(1, 2) begin
    A(1, 2) => ...
end

I have an idea to solve this: register the structs to somewhere which marks all the structs that can be destructured.

struct A
  a_1
  a_2
end

@default_pattern A

@match A(1, 2) begin
    A(1, 2) => ...
end

[thautwarm]

Final syntax:

struct A
  a_1
  a_2
end

@as_record A

@match A(1, 2) begin
    A(1, 2) => ...
end