ast, read_verilog: ownership in AST, use C++ styles for parser and lexer

[widlarizer]

The purpose of this PR is to clarify resource ownership (responsibility for construction and deletion) in the AST by distinguishing between owning pointers (std::unique_ptr<AstNode>) and non-owning pointers (AstNode*).

Core idea

struct AstNode now holds these members:

std::vector<std::unique_ptr<AstNode>> children;
std::map<RTLIL::IdString, std::unique_ptr<AstNode>> attributes;

There are no more new AstNode and corresponding delete statements present in the codebase. std::unique_ptr<AstNode> falling out of scope triggers ~AstNode, destroying its children, too. Assigning nullptr to it behaves the same way. This should eliminate AstNode allocations being reported as leaking on yosys termination by valgrind.

Damage

This implied returning std::unique_ptr<AstNode> from parse rules. Prior to this PR, the return data type for parse rules is auto-generated with the bison %union directive. The auto-generated code handling it relies on each union member type having an T& operator=(T&);, but std::unique_ptr<...> deletes its own for good reason.

That's why I refactored the parser and lexer with their C++ modes causing a greater code change.

Somehow, syntax errors now don't quote characters, so you get ERROR: syntax error, unexpected ; instead of ERROR: syntax error, unexpected ';'.

Testing

This PR has a high risk of creating new yosys segfaults in edge cases not hit by our test suite. More extensive testing will be needed

TODOs

• Source locations are temporarily completely broken. This breaks tests
• Can global variables be eliminated from the parser by moving them into the generated parser class?
• verilog_frontend.h currently includes verilog_parser.tab.hh, which is a missing Makefile dependency (make has to be rerun a couple times when building) and is probably a downgrade in compilation times
• Extend ownership in ast and parser to attribute lists and strings. Ccontainers already basically are owned pointers, but maybe should be still pointed to with std::unique_ptr to avoid accidental copies
• Go through the entire diff again and check for use-after-move patterns I may have missed
• Play around with memory and address sanitizers to see if we're cutting down on the leaks