Release 7.6.0

• For command obfuscate add option --mix-str to obfuscate the string value
• For command config add option --mixin, only avaliable mixin is str now
• Project add new attribute mixins to support option --mixin
• Change core version to r51.5
• Change spp build library version to r2
• Fix spp mode crash bugs because of the same list/dict/set constants conflicts
• Fix spp mode crash bugs because cellvars and freevars are freed even they are still used by other lambda/functions.