v.util: add a small cache for util.read_file, so reading individual source files is done just once

vlib/v/builder/compile.v

@@ -83,9 +83,12 @@ fn (mut b Builder) myfree() {
 	// for file in b.parsed_files {
 	// }
 	unsafe { b.parsed_files.free() }
+	unsafe { util.cached_read_source_file('') or {} }
 }
 
 fn (b &Builder) exit_on_invalid_syntax() {
+	// clear the source file cache, since it will not be needed anymore
+	unsafe { util.cached_read_source_file('') or {} }
 	// V should exit with an exit code of 1, when there are errors,
 	// even when -silent is passed in combination to -check-syntax:
 	if b.pref.only_check_syntax {

vlib/v/util/util.v

@@ -267,9 +267,38 @@ pub fn path_of_executable(path string) string {
 	return path
 }
 
+[heap]
+struct SourceCache {
+mut:
+	sources map[string]string
+}
+
+[unsafe]
+pub fn cached_read_source_file(path string) ?string {
+	mut static cache := &SourceCache(0)
+	if isnil(cache) {
+		cache = &SourceCache{}
+	}
+	if path.len == 0 {
+		unsafe { cache.sources.free() }
+		unsafe { free(cache) }
+		cache = &SourceCache(0)
+		return error('memory source file cache cleared')
+	}
+	// eprintln('>> cached_read_source_file path: $path')
+	if res := cache.sources[path] {
+		// eprintln('>> cached')
+		return res
+	}
+	// eprintln('>> not cached | cache.sources.len: $cache.sources.len')
+	raw_text := os.read_file(path) or { return error('failed to open $path') }
+	res := skip_bom(raw_text)
+	cache.sources[path] = res
+	return res
+}
+
 pub fn read_file(file_path string) ?string {
-	raw_text := os.read_file(file_path) or { return error('failed to open $file_path') }
-	return skip_bom(raw_text)
+	return unsafe { cached_read_source_file(file_path) }
 }
 
 pub fn skip_bom(file_content string) string {