Add data.strz directive for zero-terminated strings

A common pattern in fox32 software is this:

	data.str "Some string here" data.8 0

i.e. a zero- or NUL-terminated string. To make such strings easier to
write, introduce a new directive data.strz, which adds the zero byte
automatically:

	data.strz "Some string here"

src/fox32.pest

@@ -19,12 +19,14 @@ data = {
     data_half |
     data_word |
     data_str  |
+    data_strz |
     data_fill
 }
 data_byte = { "data.8" ~ operand_value }
 data_half = { "data.16" ~ operand_value }
 data_word = { "data.32" ~ operand_value }
 data_str  = { "data.str" ~ immediate_str }
+data_strz = { "data.strz" ~ immediate_str }
 data_fill = { "data.fill" ~ operand_value ~ "," ~ operand_value }
 
 constant = { "const" ~ constant_name ~ operand_value }

src/main.rs

@@ -291,6 +291,7 @@ enum AstNode {
     DataHalf(u16),
     DataWord(u32),
     DataStr(String),
+    DataStrZero(String),
     DataFill {
         value: u8,
         size: u32,
@@ -630,6 +631,10 @@ fn parse_data(pair: pest::iterators::Pair<Rule>) -> AstNode {
             let string = pair.into_inner().next().unwrap().into_inner().next().unwrap().as_str();
             AstNode::DataStr(string.to_string())
         },
+        Rule::data_strz => {
+            let string = pair.into_inner().next().unwrap().into_inner().next().unwrap().as_str();
+            AstNode::DataStrZero(string.to_string())
+        },
         Rule::data_fill => {
             let value = {
                 let ast = parse_operand(pair.clone().into_inner().next().unwrap(), false);
@@ -982,6 +987,11 @@ fn assemble_node(node: AstNode) -> AssembledInstruction {
         AstNode::DataStr(string) => {
             return string.as_bytes().into();
         },
+        AstNode::DataStrZero(string) => {
+            let mut bytes: Vec<u8> = string.as_bytes().into();
+            bytes.push(0);
+            return bytes.into();
+        },
         AstNode::LabelOperand {name, size, is_relative} => {
             // label is used on its own, not as an operand:
             // LabelOperand was previously only checked as part of operands