1. Slot is a new mechanism for storing variables during execution
which is more convenient than alt.stack. This commit implements
support for slot opcodes in both vm and compiler.
2. Remove old alt.stack opcodes.
3. Do not process globals at the start of every function, but instead
load them single time at main.
Rename THROWIFNOT to ASSERT, add ABORT opcode.
ABORT cannot be caught, but the implementation should be postponed until
exception handling is implemented.
In NEO3 SYSCALL opcode has 4-byte ID parameter.
This commit removes support for string-based syscalls and
changes SYSCALL's parameter to be fixed 4-byte value.
