Add value directives and strings

.string, .zstring, .u64, .u32, .u16, .u8 are used before an ImmValue to
determine how the memory should be laid out for that value.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

examples/strings.asm

@@ -0,0 +1,45 @@
+data $0x1000 .. $0x1100 {
+    zstr: .zstring  "This is a zero-terminated string"
+
+    .export zstr
+}
+
+code $0x0 {
+    zstr_len:
+        storeimm32 %r15, $0xFF
+        storeimm32 %r16, $1
+        storeimm64 %r20, zstr_next
+        storeimm64 %r21, exit_zstr
+
+        regcopy %r10, %r00
+
+    zstr_next:
+        load %r11, %r10
+        and %r11, %r15
+        cmpeq %r11, %null
+        jnz %r21
+        add %r10, %r16
+        jmp %r20
+
+    main:
+        storeimm64 %r05, zstr_len
+        storeimm64 %r00, zstr
+        jmp %r05
+
+    exit_zstr:
+        regcopy %status, %r10
+        halt
+    
+        ineg %r00
+        add %r10, %r00
+        regcopy %status, %r10
+
+    end:
+        halt
+
+    .export main
+}
+
+meta {
+    entry: main
+}