More usability woes

[p0nce]

I tried to modify njson to use more numem:

• .length for vector (also: slength for signed length?)
• .ptr for string
• for shared_ptr and unique_ptr, .ptr gets repeated a lot, at any point of use. Not sure how alias this can settle that.
• ref inout(T) for vector index and slices, currently only mutable supported. Tricky with atomics.
• ref inout(T) for string index and slices

Note during yxml development:

• unique_ptr!A not naturally covariant with a B class that would descend from A. This is quite complicated, when involving vector!unique_ptr after... ouch
• nogc_delete crashes if the class instance access its surrounding frame (not a static class). So this shall be documented, I don't see a way out. (EDIT: this is related to std.conv.emplace segfaults for nested types dlang/phobos#10125)
• unique_ptr can't be copied:

  unique_ptr!A a = unique_new!A();
  vector!(unique_ptr!A) v;
  v ~= a; // fail
  v ~= unique_new!A(); // but this works

  perhaps an explicit ".appendMove" operation?
• vector!(unique_new!Class) owns every element. But vector!(Struct*) and vector!Class also own every element. It just bypass the whole move constraint and is more usable for now than vector!(unique_ptr!Class)