0.6.0

• BREAKING: Deprecate Cell::find_with_attr in favor of Cell::find_with_unique_attr. The former is error-prone because pass logic might implicitly assume that there is only one port with a particular attribute.
• BREAKING: Redesign the ir::Rewriter interface to take all the rewrite maps when constructing the ir::Rewriter struct.
• Merge the logic of compile-ref pass into compile-invoke so that ref cells can be invoked.
• The guard! macro supports parsing complex guard expressions that use logical connectives and comparison operators.
• The calyx library no longer exposes any methods and should not be depended upon. Instead, the new calyx-backend crate provides the code needed to emit Verilog from Calyx.