- Comments on how to handle the semantics of multiple PRE invocations
  - and design option of splitting the PRE into PRE and BQE (Buffering
  and Queueing Engine)
  - added PRE as argument to Ingress and Egress
  - added missing 'in' qualifiers and random distribution
  - add size types instead of indefinite int
  - update with @mbudiu-vmw comments to pass through compiler
  - ActionProfile and ActionSelector externs

This commit refactors bmv2 backend to support multiple pipelien
architectures. JsonConverter is refactored into a pass manager
which is more conformed to the rest of the compiler design.

Added files:
backend.cpp       - the new backend pass manager
convert*.cpp      - compiler passes for control, parser, deparser,
                    externs and headers
inferArchitecture - a new frontend pass to parse the architecture p4
                    into a data object which is then used by json
                    generator
model.cpp         - a v2model that oriented for p4-16 IR

TODO:
tov1model         - a pass to convert v2model to v1model to execute
                    on simple-switch

Also fixes issues with decl handling and match types.

BMV2 supports field_aliases to facilitate using different names
in the P4 programs from the names supported by the target.
For PSA and P4D2 we define a different set of metadata and a map
file to map to bmv2 simple_switch (aka v1model). The name map name
is currently hardcoded, and it should be set by the compiler driver
as it knows which architecture and which target.

Also removes the force_arith field in json, since it is no longer used.

This started initially as a pass to rename metadata in the program.
It seems to not be necessary, so the code is #ifdef-ed out. We should
remove it in the final version if we find that it is really not
needed.

* generate only one header instance

* disable field aliases until we figure out how to handle multiple aliases to the same standard metadata field

…ess (#12)

…e not the same

- refactor meta_info, program and enum generation

…y simple switch

Allows access to bmv2 simple switch intrinsic and queueing metadata
for programs that use the v1model. While these metadata would be
available by defining separate metadata with the correct names, it is
desirable that the functionality is exposed in the architecture
definition. This patch will work with any metadata that has fields
renamed.