Start doing name lookup for references (#2472)

When binding a name, add it to name lookup. On NameReference nodes, use name lookup.

- Switches from "identifiers" to the more generic "strings". Not strictly necessary here, but it's the overall direction I think we've agreed upon and wanted to do it while building more support out.
- Starts doing deduplication of strings.
- On BindName, registers names with name lookup.
- Does name lookup based on the deduplicated string.
  - Per discussion with zygoloid, design is intended to be constant-time lookup regardless of the number of parent scopes.
- Adds scopes so that we can track names which will be deregistered from lookup.

toolchain/semantics/semantics_ir.cpp

@@ -87,13 +87,6 @@ auto SemanticsIR::Print(llvm::raw_ostream& out) const -> void {
   }
   out << "},\n";
 
-  out << "identifiers = {\n";
-  for (int32_t i = 0; i < static_cast<int32_t>(identifiers_.size()); ++i) {
-    out.indent(Indent);
-    out << SemanticsIdentifierId(i) << " = \"" << identifiers_[i] << "\";\n";
-  }
-  out << "},\n";
-
   out << "integer_literals = {\n";
   for (int32_t i = 0; i < static_cast<int32_t>(integer_literals_.size()); ++i) {
     out.indent(Indent);
@@ -102,6 +95,13 @@ auto SemanticsIR::Print(llvm::raw_ostream& out) const -> void {
   }
   out << "},\n";
 
+  out << "strings = {\n";
+  for (int32_t i = 0; i < static_cast<int32_t>(strings_.size()); ++i) {
+    out.indent(Indent);
+    out << SemanticsStringId(i) << " = \"" << strings_[i] << "\";\n";
+  }
+  out << "},\n";
+
   out << "node_blocks = {\n";
   for (int32_t i = 0; i < static_cast<int32_t>(node_blocks_.size()); ++i) {
     out.indent(Indent);