Add syntax for package and library (#792)

Doesn't add much logic, only takes advantage of parser structure for the ordering enforcement.

Note import_nonexistent tests should probably fail, but writing import tests needs a chain of functionality, and I figured I'd just start adding some to validate the syntax (not adding existent imports because that'd require multi-file structure).

executable_semantics/syntax/parser.ypp

@@ -65,7 +65,7 @@
 %code requires {
   #include <optional>
 
-  #include "executable_semantics/ast/abstract_syntax_tree.h"
+  #include "executable_semantics/ast/ast.h"
   #include "executable_semantics/ast/declaration.h"
   #include "executable_semantics/ast/expression.h"
   #include "executable_semantics/ast/function_definition.h"
@@ -93,6 +93,11 @@
 %token <std::string> sized_type_literal
 %token <std::string> string_literal
 %type <std::string> designator
+%type <std::pair<LibraryName, bool>> package_directive
+%type <LibraryName> import_directive
+%type <std::vector<LibraryName>> import_directives
+%type <std::string> optional_library_path
+%type <bool> api_or_impl
 %type <BisonWrap<Ptr<const Declaration>>> declaration
 %type <BisonWrap<Ptr<const FunctionDefinition>>> function_declaration
 %type <BisonWrap<Ptr<const FunctionDefinition>>> function_definition
@@ -133,6 +138,7 @@
 %token
   // Most tokens have their spelling defined in lexer.lpp.
   AND
+  API
   ARROW
   AUTO
   AWAIT
@@ -156,13 +162,17 @@
   FN
   FNTY
   IF
+  IMPL
+  IMPORT
   LEFT_CURLY_BRACE
   LEFT_PARENTHESIS
   LEFT_SQUARE_BRACKET
+  LIBRARY
   MATCH
   MINUS
   NOT
   OR
+  PACKAGE
   PERIOD
   PLUS
   RETURN
@@ -217,8 +227,42 @@
 
 %start input
 %%
-input: declaration_list
-    { parsed_program = $1; }
+input: package_directive import_directives declaration_list
+    {
+      parsed_program = AST({.package = $1.first,
+                            .is_api = $1.second,
+                            .imports = std::move($2),
+                            .declarations = std::move($3)});
+    }
+;
+package_directive:
+  PACKAGE identifier optional_library_path api_or_impl SEMICOLON
+    { $$ = {LibraryName({.package = $2, .path = $3}), $4}; }
+;
+import_directive:
+  IMPORT identifier optional_library_path SEMICOLON
+    { $$ = LibraryName({.package = $2, .path = $3}); }
+;
+import_directives:
+  // Empty
+    { $$ = std::vector<LibraryName>(); }
+| import_directives import_directive
+    {
+      $$ = std::move($1);
+      $$.push_back($2);
+    }
+;
+optional_library_path:
+  // Empty
+    { $$ = ""; }
+| LIBRARY string_literal
+    { $$ = $2; }
+;
+api_or_impl:
+  API
+    { $$ = true; }
+| IMPL
+    { $$ = false; }
 ;
 expression:
   identifier