Remove conflict between function and struct names

There was a conflict between the function `bcon_extract_ctx` and the raw struct name (which is typedef'ed to `bcon_extract_ctx_t` anyway). A similar situation happened for `bcon_append_ctx`. This change simply renames the raw names, since they are never used.
Update to use underscore prefixes for structs
And now with a `_t` suffix
Merge pull request #75 from itay/patch-1

Remove conflict between function and struct names