Rewrite grammar and parser

This commit is a complete rewrite of the Smithy IDL parser and grammar.
It is intended to clear up the intention of how the IDL works in
practice by making the grammar embed things like how control statements
come before metadata, which come before namespace, etc.. It also
clarifies how different node_value locations have different semantics
(e.g., keys in objects are not resolved against keywords like
true and do not perform relative shape ID resolution nor do they support
text blocks). Many other fixes have been made to the grammar too,
including specifying where whitespace is allowed and where line breaks
are required. This makes the grammar more verbose but also more accurate
and in line with what was already implemented.

With these changes, control statement keyus, metadata keys, and object keys
must now be quoted if they contain ".", "#", or "$". In practice, the
model parser already enforced these constraints in some places.

docs/source/1.0/spec/core/model-metadata.rst

@@ -5,7 +5,10 @@ Model metadata
 ==============
 
 :dfn:`Metadata` is a schema-less extensibility mechanism that can be applied
-to a model using a :ref:`metadata statement <metadata-statement>`.
+to a model using a :ref:`metadata statement <metadata-statement>`. For
+example, metadata is used to define :ref:`validators <validation>` and
+:ref:`suppressions <suppression-definition>` that are applied to the entire
+model.
 
 .. contents:: Table of contents
     :depth: 1
@@ -19,26 +22,24 @@ to a model using a :ref:`metadata statement <metadata-statement>`.
 Metadata statement
 ------------------
 
-The metadata statement is used to attach arbitrary :ref:`metadata <metadata>`
-to a model. Metadata statements MUST appear before a namespace statement
-or any shapes are defined.
+Metadata statements MUST appear before any namespace statement or any shapes
+are defined. Metadata is defined by the following ABNF:
 
 .. productionlist:: smithy
-    metadata_statement:"metadata" `metadata_key` "=" `metadata_value`
-    metadata_key:`text`
-    metadata_value:`node_value`
+    metadata_section   :*(`metadata_statement`)
+    metadata_statement :"metadata" `ws` `node_object_key` `ws` "=" `ws` `node_value` `br`
 
 .. code-block:: smithy
     :caption: Example
 
-    metadata example.string1 = "hello there"
-    metadata example.string2 = 'hello there'
-    metadata example.bool1 = true
-    metadata example.bool2 = false
-    metadata example.number = 10
-    metadata example.array = [10, true, "hello"]
-    metadata example.object = {foo: "baz"}
-    metadata example.null = null
+    metadata exampleString = "hello there"
+    metadata "example.string2" = 'hello there'
+    metadata bool1 = true
+    metadata bool2 = false
+    metadata number = 10
+    metadata array = [10, true, "hello"]
+    metadata object = {foo: "baz"}
+    metadata null = null
 
 
 .. _merging-metadata: