strings: add Builder.drain_builder/1 utility method

vlib/strings/builder.c.v

@@ -62,6 +62,17 @@ pub fn (mut b Builder) write(data []byte) ?int {
 	return data.len
 }
 
+// drain_builder writes all of the `other` builder content, then re-initialises
+// `other`, so that the `other` strings builder is ready to receive new content.
+[manualfree]
+pub fn (mut b Builder) drain_builder(mut other Builder, other_new_cap int) {
+	b.write(other) or { panic(err) }
+	unsafe { other.free() }
+	other = new_builder(other_new_cap)
+}
+
+// byte_at returns a byte, located at a given index `i`.
+// NB: it can panic, if there are not enough bytes in the strings builder yet.
 [inline]
 pub fn (b &Builder) byte_at(n int) byte {
 	return unsafe { (&[]byte(b))[n] }

vlib/v/gen/c/cgen.v

@@ -342,18 +342,10 @@ pub fn gen(files []&ast.File, table &ast.Table, pref &pref.Preferences) string {
 		for file in files {
 			global_g.file = file
 			global_g.gen_file()
-
-			global_g.inits[file.mod.name].write(global_g.init) or { panic(err) }
-			unsafe { global_g.init.free() }
-			global_g.init = strings.new_builder(100)
-
-			global_g.cleanups[file.mod.name].write(global_g.cleanup) or { panic(err) }
-			unsafe { global_g.cleanup.free() }
-			global_g.cleanup = strings.new_builder(100)
-
-			global_g.global_inits[file.mod.name].write(global_g.global_init) or { panic(err) }
-			unsafe { global_g.global_init.free() }
-			global_g.global_init = strings.new_builder(100)
+			global_g.inits[file.mod.name].drain_builder(mut global_g.init, 100)
+			global_g.cleanups[file.mod.name].drain_builder(mut global_g.cleanup, 100)
+			global_g.global_inits[file.mod.name].drain_builder(mut global_g.global_init,
+				100)
 		}
 		global_g.timers.start('cgen unification')
 	}