Complete rewrite.

This makes pear significantly more robust: no more "remonad" warnings
and special-casing! The code is significantly smaller, easier to
understand, and easier to manager.

There is one major open question: How should input be passed to macros?

At present, it is not, and macros rely on the input being named `input`.
This approach will not work for macro_rules macros as it requires
breaking hygiene, and so a better approach is required.

One potential approach is to pass the input as the first parameter to
any macro prefixed with `pear`.

Cargo.toml

@@ -2,10 +2,10 @@
 members = [
   "lib/",
   "codegen/",
-  "examples/http",
   "examples/ini",
-  "examples/media_type",
-  "examples/exprs",
+  # "examples/http",
+  # "examples/media_type",
+  # "examples/exprs",
   "examples/parens",
   "examples/json",
   "examples/uri",

codegen/Cargo.toml

@@ -6,7 +6,12 @@ description = "A (codegen) pear is a fruit."
 license = "MIT/Apache-2.0"
 
 [lib]
-plugin = true
+proc-macro = true
+
+[dependencies]
+quote = "*"
+proc-macro2 = { version = "*", features = ["nightly"] }
+syn = { version = "0.13.2", features = ["full", "extra-traits", "visit-mut"] }
 
 [build-dependencies]
 yansi = "0.4"