implement the throw statement

This was `die` for a short while in a branch, but #65 says we
should call it `throw`.

Also introduces the Val::Exception type, which is the only
thing you can throw. (In the branch it was Val::Str, but that's
deprecated now with extreme prejudice.)

lib/_007/Parser/Actions.pm

@@ -148,6 +148,10 @@ class _007::Parser::Actions {
         make Q::Statement::Return.new(:expr($<EXPR> ?? $<EXPR>.ast !! Val::None.new));
     }
 
+    method statement:throw ($/) {
+        make Q::Statement::Throw.new(:expr($<EXPR> ?? $<EXPR>.ast !! Val::None.new));
+    }
+
     method statement:if ($/) {
         my %parameters = $<xblock>.ast;
         %parameters<else> = $<else> :exists

lib/_007/Parser/Syntax.pm

@@ -89,6 +89,11 @@ grammar _007::Parser::Syntax {
                 unless $*insub;
         }
     }
+
+    token statement:throw {
+        throw [<.ws> <EXPR>]?
+    }
+
     rule statement:if {
         if <xblock>
         [  else

lib/_007/Q.pm

@@ -649,6 +649,18 @@ class Q::Statement::Return does Q::Statement {
     }
 }
 
+class Q::Statement::Throw does Q::Statement {
+    has $.expr = Val::None.new;
+
+    method run($runtime) {
+        my $value = $.expr ~~ Val::None ?? Val::Exception.new(:message("Died")) !! $.expr.eval($runtime);
+        die X::TypeCheck.new(:got($value), :excpected(Val::Exception))
+            if $value !~~ Val::Exception;
+
+        die $value.message;
+    }
+}
+
 class Q::Statement::Sub does Q::Statement does Q::Declaration {
     has $.identifier;
     has $.traitlist = Q::TraitList.new;

lib/_007/Runtime/Builtins.pm

@@ -358,6 +358,7 @@ class _007::Runtime::Builtins {
             Val::Str,
             Val::Sub,
             Val::Type,
+            Val::Exception,
         ;
 
         for @val-types -> $type {
@@ -413,6 +414,7 @@ class _007::Runtime::Builtins {
             Q::Statement::My,
             Q::Statement::Return,
             Q::Statement::Sub,
+            Q::Statement::Throw,
             Q::Statement::While,
             Q::StatementList,
             Q::Term::Array,

lib/_007/Val.pm

@@ -163,3 +163,20 @@ class Val::Macro is Val::Sub {
 
     method Str { "<macro {$.name}{$.pretty-parameters}>" }
 }
+
+class Val::Exception does Val {
+    has Str $.message;
+
+    method Str {
+        my $message = Val::Str.new(:value($.message));
+        return "Exception \{message: {$message.quoted-Str}\}";
+    }
+
+    method quoted-Str {
+        self.Str
+    }
+
+    method truthy {
+        ?%.properties
+    }
+}