Umka 0.9

• Strings are now immutable
• Interface-typed map keys are no longer allowed
• New dynamic array handling functions: copy, insert
• New math functions: ceil, floor and the % operator for reals
• New date/time handling functions: std.localtime, std.gmtime, std.mktime, std.timestr
• New flow control function: exit
• Improved I/O functions: sprintf, std.fread, std.fwrite
• C API for garbage collection and map items access
• Source file and line in stack traces
• Bug fixes