Native JSON support

[nedpals]

This issue tracks the current progress of native JSON support that will replace the CJSON-based json module. Replaces #309.

PR's

• Add x/json2 (vlib: Add x/json2 #6336)
• Add i64 and f32 support (x.json2: add fast_raw_decode; support for i64 and f32 #6586)
• Have json2.decode return ?T (json2: decode fn returns ?T; add new tests #6933)
• Encoder/decoder codegen

Related Issues

Just a series of issues whose suggestions can be a good addition to the codegen.

• Implement omitifempty (json needs attribute like Go's omitempty #6571, [Enhancement] json module can not decode on some case on array. #6344)
• Support for aliased types (Json: Cannot use alias in struct fields #6225)