support enums with more than 256 variants in derive macro (#4370)

* support enums with more than 256 variants in derive macro

This addresses #4361.  Technically, we now support up to 2^32 variants, which is
the maximum for the canonical ABI.  In practice, though, the derived code for
enums with even just 2^16 variants takes a prohibitively long time to compile.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

* simplify `LowerExpander::expand_variant` code

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Cargo.lock

@@ -450,6 +450,15 @@ dependencies = [
  "cc",
 ]
 
+[[package]]
+name = "component-macro-test"
+version = "0.1.0"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
 [[package]]
 name = "console"
 version = "0.15.0"
@@ -3413,6 +3422,7 @@ dependencies = [
  "anyhow",
  "async-trait",
  "clap 3.1.15",
+ "component-macro-test",
  "criterion",
  "env_logger 0.9.0",
  "filecheck",