keep arrays as faster alternative to lookup maps

tested with new tool

vlib/v/fmt/fmt.v

@@ -31,9 +31,9 @@ pub mut:
 	single_line_if     bool
 	cur_mod            string
 	did_imports        bool
-	import_pos         int // position of the imports in the resulting string for later autoimports insertion
-	auto_imports       map[string]bool   // potentially hidden imports(`sync` when using channels) and preludes(when embedding files)
-	used_imports       map[string]bool   // to remove unused imports
+	import_pos         int               // position of the imports in the resulting string for later autoimports insertion
+	auto_imports       []string          // potentially hidden imports(`sync` when using channels) and preludes(when embedding files)
+	used_imports       []string          // to remove unused imports
 	import_syms_used   map[string]bool   // to remove unused import symbols
 	mod2alias          map[string]string // for `import time as t`, will contain: 'time'=>'t'
 	mod2syms           map[string]string // import time { now } 'time.now'=>'now'
@@ -112,9 +112,7 @@ pub fn (mut f Fmt) process_file_imports(file &ast.File) {
 			f.import_syms_used[sym.name] = false
 		}
 	}
-	for mod in f.file.auto_imports {
-		f.auto_imports[mod] = true
-	}
+	f.auto_imports = file.auto_imports
 }
 
 //=== Basic buffer write operations ===//
@@ -319,8 +317,8 @@ pub fn (mut f Fmt) mark_types_import_as_used(typ ast.Type) {
 pub fn (mut f Fmt) mark_import_as_used(name string) {
 	mod, sym := name.rsplit_once('.') or { '', name }
 	f.import_syms_used[sym] = true
-	if mod != '' {
-		f.used_imports[mod] = true
+	if mod != '' && mod !in f.used_imports {
+		f.used_imports << mod
 	}
 }
 
@@ -331,7 +329,7 @@ pub fn (mut f Fmt) imports(imports []ast.Import) {
 	f.did_imports = true
 	mut processed_imports := map[string]bool{}
 	for imp in imports {
-		if !f.used_imports[imp.mod] && f.auto_imports[imp.mod] {
+		if imp.mod in f.auto_imports && imp.mod !in f.used_imports {
 			// Skip hidden imports like preludes.
 			continue
 		}
@@ -1954,7 +1952,9 @@ pub fn (mut f Fmt) call_expr(node ast.CallExpr) {
 					mod: node.left.name
 					alias: node.left.name
 				}
-				f.used_imports[node.left.name] = true
+				if node.left.name !in f.used_imports {
+					f.used_imports << node.left.name
+				}
 			}
 		}
 		f.expr(node.left)