Fix codegen

[hugopl]

Codegen is not tested at all, so before think about fix it is better to add tests for it.

The plan is:

• Let golden test run against codegen visitor.
• Compare codegen output with expected golden test output.
• Fix the code gen.

I had some time today and made the tests run against the codegen, still in a "it worked" state, very draft, but I'm pushing it here because if someone with time is interested in fix this there's something to start on.

All codegen tests are tagged with codegen, currently if you run: crystal spec --tag codegen --fail-fast you will reproduce issue #87

$ crystal spec --tag codegen --fail-fast
In spec/integration/4wtigu49-liquid-codegen-test.cr:4:3

 4 | {{ run "../../src/process", "/home/hugo/src/pet/liquid.cr/spec/integration/x6qfc7po-test.liquid", "io", "context" }}
     ^
Error: expanding macro


In src/process.cr:7:5

 7 | tpl.to_code io, STDOUT, context
         ^------
Error: instantiating 'Liquid::Template#to_code(String, IO::FileDescriptor, (String | Nil))'


In src/liquid/template.cr:60:12

 60 | root.accept visitor
           ^-----
Error: instantiating 'Liquid::Block::Root#accept(Liquid::CodeGenVisitor)'


In src/liquid/blocks/block.cr:12:15

 12 | visitor.visit self
              ^----
Error: instantiating 'Liquid::CodeGenVisitor#visit(Liquid::Block::Root)'


In src/liquid/codegen_visitor.cr:49:28

 49 | node.children.each &.accept self
                           ^-----
Error: instantiating 'Liquid::Block::Node+#accept(Liquid::CodeGenVisitor)'


In src/liquid/blocks/block.cr:12:15

 12 | visitor.visit self
              ^----
Error: instantiating 'Liquid::CodeGenVisitor#visit(Liquid::Block::Node+)'


In src/liquid/codegen_visitor.cr:58:76

 58 | Liquid::Block::ExpressionNode.new("#{escape node.value.var}")))
                                                             ^--
Error: undefined method 'var' for Liquid::Expression
F

Failures:

  1) Golden Liquid Tests liquid.golden.abs_filter negative float (codegen)
     Failure/Error: $?.exit_code.should eq(0)

       Expected: 0
            got: 1

     # spec/integration/integration_spec.cr:43

Aborted!
Finished in 626.58 milliseconds
1 examples, 1 failures, 0 errors, 0 pending

Failed examples:

crystal spec spec/integration/integration_spec.cr:1 # Golden Liquid Tests liquid.golden.abs_filter negative float (codegen)

[straight-shoota]

I'd suggest moving the formatter test to a separate job. It's unrelated to the specs. And it doesn't need to run in matrix, the behaviour will be identical regardless of which OS it runs on. This might be out of scope of this PR, of course.

[hugopl]

I added ameba github action in another PR, so it must do the format check.

Now we just need someone to review, approve and merge both PR's