freestanding: ptr_str, string concat, struct declaration

* compiler: extract c_common_macros from c_headers, and use it in bare_c_headers too. Support for ptr_str and string concatenation in -freestanding mode.

* Add tests for structs and string concatenation in -freestanding mode .

* Move check_string_add_works to string/string.v .

vlib/builtin/bare/.checks/.gitignore

@@ -1 +1,5 @@
 checks
+linuxsys/linuxsys
+string/string
+consts/consts
+structs/structs

vlib/builtin/bare/.checks/checks.v

@@ -26,6 +26,7 @@ fn main() {
 	vcheck("linuxsys")
 	vcheck("string")
 	vcheck("consts")
+	vcheck("structs")
 	exit(0)
 }
 

vlib/builtin/bare/.checks/string/string.v

@@ -39,12 +39,24 @@ fn check_str_clone() {
 	assert c == "-6789"
 }
 
+fn check_string_add_works(){
+  abc := 'abc'
+  combined := 'a' + 'b' + 'c'
+  assert abc.len == combined.len
+  assert abc[0] == combined[0]
+  assert abc[1] == combined[1]
+  assert abc[2] == combined[2]
+  assert abc[0] == `a`
+  assert abc == combined
+}  
+
 fn main () {
 	mut fails := 0
 	fails += forkedtest.normal_run(check_string_eq, "check_string_eq")
 	fails += forkedtest.normal_run(check_i64_tos, "check_i64_tos")
 	fails += forkedtest.normal_run(check_i64_str, "check_i64_str")
 	fails += forkedtest.normal_run(check_str_clone, "check_str_clone")
+	fails += forkedtest.normal_run(check_string_add_works,    "check_string_add_works")
 	assert fails == 0
 	sys_exit(0)
 }

vlib/builtin/bare/.checks/structs/structs.v

@@ -0,0 +1,42 @@
+module main
+import forkedtest
+
+struct SimpleEmptyStruct{
+}
+
+struct NonEmptyStruct{
+  x int
+  y int
+  z int
+}
+
+fn check_simple_empty_struct(){  
+  s := SimpleEmptyStruct{}
+  addr_s := &s
+  str_addr_s := ptr_str( addr_s )
+  assert !isnil(addr_s)
+  assert str_addr_s.len > 3
+  println(str_addr_s)
+}
+
+fn check_non_empty_struct(){  
+  a := NonEmptyStruct{1,2,3}
+  b := NonEmptyStruct{4,5,6}
+  assert sizeof(NonEmptyStruct) > 0
+  assert sizeof(SimpleEmptyStruct) < sizeof(NonEmptyStruct)
+  assert a.x == 1
+  assert a.y == 2
+  assert a.z == 3
+  assert b.x + b.y + b.z == 15
+  assert ptr_str(&a) != ptr_str(&b)
+  println('sizeof SimpleEmptyStruct:' + i64_str( sizeof(SimpleEmptyStruct) , 10 ))
+  println('sizeof NonEmptyStruct:' + i64_str( sizeof(NonEmptyStruct) , 10 ))
+}
+
+fn main(){
+	mut fails := 0
+	fails += forkedtest.normal_run(check_simple_empty_struct, "check_simple_empty_struct")
+	fails += forkedtest.normal_run(check_non_empty_struct,    "check_non_empty_struct")
+	assert fails == 0
+	sys_exit(0)
+}