lib-sieve: optimized compilation of tests that yield constant results (i.e. known at compile tme), such as true and false.
If the result of a test is known at compile time, it is optimized away. If an if-command depends on an entirely constant test, it is optimized away as well,
causing only the 'true' sub-block to be compiled.

- Changed operand read functions to prove exec status return codes in stead of boolean.
- Restructured optional-operand, stringlist and string-match APIs to provide execution status (error code) upon failure.
- Corruption errors are only produced when the binary is actually corrupt. Normal failures should never cause a recompile attempt anymore.
- Simplified optional-operand API.
- Cleaned up test command implementations that use the string-match API.

Substituted mail_get_headers for mail_get_headers_utf8 for those occasions where utf8 is of no concern.

Working towards improving the handling of currupt binaries: defined multiple exit codes for execution functions and defined trace macro for reporting binary corruptions.

Implemented support for overriding default argument implementations of number, string and string-list.

Added support for reading an entire stringlist into memory. Also fixed various identical bugs in stringlist-related error handling.

Properly implemented the code dumper. Dumps are now printed in a stream. The individual opcode and operand implementations no longer use printf()s.

Exported sieve-code-dumper from sieve-interpreter containing all code dumping related implementation. Now to remove all printfs....

Grouped runtime parameters into a single runtime environment and started implementation of result composition/execution.

Changed validator's command syntax validation such that command implementations don't have to call the argument, test and block validation functions explicitly.

Changed argument validation to record the first positional argument into the command context by default. Also furter improved validator's error handling.