Fixed: `is_a?` gave an incorrect result when testing a class against a generic type which that class inherits. Related to #268

Swapping parameters of `alias_method` to match Ruby

Wrapper to LLVM round, sign for numbers, sqrt, cis, ceil, floor and round for complex numbers

Added basic functionality for complex numbers

Fixed #130: `require "`foo"` always looks up in CRYSTAL_PATH, never relative to the current file. `require "./foo"` is how you require relative to the current file.