compiler, language, runtime: Separate Parser Type From Instances

In P4, parsers are considered types. Those parsers are instantiated.
The instantiated parsers are values. Previously, gp4 treated a parser
type and a parser value as identical. This PR makes that difference
clear _and_ sets the stage for the future.

TODO: Make the same distinction between control and action types and
values.

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

Tests/p4rseTests/CodeGenTests/CodeGen.swift

@@ -37,7 +37,7 @@ func shrink(_ from: String) -> String {
     parser main_parser() {
        state start {
            true;
-           transition start;
+           transition accept;
        }
     };
     """
@@ -59,7 +59,6 @@ func shrink(_ from: String) -> String {
         }
     ]
     """)
-
   #expect(expected == cg.getGeneratedCode())
 }