Settled on UTF-8 for internal encoding of strings.

In this commit, this means replacing nchar by char
everywhere, and removing the MOZART_STR macro.
Reviewed atom table implementation with UTF-8.
Reviewed VM library code with UTF-8 in mind.
Reviewed boostenv and boosthost with UTF-8 in mind.
Merge pull request #69 from sjrd/refactor/issue-55-settle-on-utf8

Definitively settle on utf8 for the internal encoding