Fix a bug of slimux and add two feature to scheme

As the commit log shows:
1. fix a potential bug: slimux might fail if we send a *very long* text.
2. Scheme feature enhanced.
    a) support for normal SlimuxSendXXX function. This is for those who don't want to use other slimux_scheme utilities.
    b) add a new but simple feature: evaluate the whole buffer. This could be used to load the whole file into scheme.

BTW:
I'm quite new to github pull request. Please inform me if I'm doing anything wrong.