Remove automatic IVM writing from raw writers?

[zslayton]

          Now that there's an application-level writer that can automate calling a `write_ivm()` method, I think we could remove the automatic functionality from the raw-level writers.

There's a minor design question around what to do for the 1.0 text writer. When the stream starts, it could leave off the $ion_1_0. However, if the user or application writer is calling write_ivm() to reset the encoding context, it could not leave it out. We could either have it always include a $ion_1_0 (unnecessary but pretty harmless) or we could pass an implicit_ivm_permitted: bool that other writers could just ignore.

Originally posted by @zslayton in #745 (comment)