Implement STRFMT and more printf-like format specifiers for string interpolation (#646)

Fixes #570
Fixes #178

Use errors for inapplicable format spec flags instead of -Wstring-format

test/asm/string-formatting.asm

@@ -0,0 +1,15 @@
+n equ 300
+m equ -42
+f equ -123.0456
+s equs "hello"
+
+	printt "<{ -6d:n}> <{+06u:n}> <{5x:n}> <{#16b:n}>\n"
+	printt "<{u:m}> <{+3d:m}> <{#016o:m}>\n"
+	printt "<{f:_PI}> <{06f:f}> <{.10f:f}>\n"
+	printt "<{#-10s:s}> <{10s:s}>\n"
+
+foo: macro
+	printt "<{\1}>\n"
+endm
+
+	foo  -6d:n ; space is trimmed