adds metadata remapping

[cc10512]

Defines the mapping for the metadata defined in p4d2model to the bmv2
simple switch intrinsic/standard metadata and provides a read function
for the file that returns the remapping (map of string to string).

We need to hook this up with p4c such that it knows which file to load
depending on the architecture and the target. And probably just simply
visit all the metadata declarations in the program and add a name
annotation to remap the name. I hope that fields can take qualified
names.

[vgurevich]

Why do you map type.field into instance.field? In other words why this mapping instead of
parser_input_t.ingress_port -> stadnard_metadata_t.ingress_port

[cc10512]

because simple switch calls it standard_metadata and v1model actually renames standard_metadata_t to standard_metadata with a @name annotation

[vgurevich]

I think the problem might be that in the original compiler (and specifically in p4-hlir) standard_metadata was instantiated automatically, without the corresponding "header_type" being explicitly declared. Similarly, in simple_switch standard_metadata is instantiated in a very "back-door"-ish way without the type being declared first.

But going forward, I suggest we should be more formal about these things, otherwise it is too confusing.

[hanw]

Thanks for writing this up, I will merge it and improve the code on my branch.