simplify managing the features in the crate

[gabhijit]

Currently the crate supports three features python-bindings, logging and (soon) wasm. Some of these features are mutually incompatible and hence mutually exclusive as well. Handling these features has resulted in code with a large number of #[cfg(...)] types statement.

This can be perhaps simplified, or the feature level code is kept limited to a couple of files and some simple intuitive macros.

[gabhijit]

A lot of cleanup was performed by #71 .

[gabhijit]

Further fixes by #72 . Closing this.