Drop dependency on mutex_m

It doesn't really save much effort, and it's being
extracted as a gem which cause issues for projects
like spring that often are loaded before bundler.

lib/spring/watcher/abstract.rb

@@ -1,5 +1,4 @@
 require "pathname"
-require "mutex_m"
 
 module Spring
   module Watcher
@@ -9,13 +8,10 @@ module Watcher
     #   IO.select([watcher]) # watcher is running in background
     #   watcher.stale? # => true
     class Abstract
-      include Mutex_m
-
       attr_reader :files, :directories, :root, :latency
 
       def initialize(root, latency)
-        super()
-
+        @mutex       = Mutex.new
         @root        = File.realpath(root)
         @latency     = latency
         @files       = {}
@@ -59,7 +55,7 @@ def add(*items)
           end
         end
 
-        synchronize {
+        @mutex.synchronize do
           items.each do |item|
             if item.directory?
               directories[item.realpath.to_s] = true
@@ -75,7 +71,7 @@ def add(*items)
           end
 
           subjects_changed
-        }
+        end
       end
 
       def stale?

spring.gemspec

@@ -15,8 +15,6 @@ Gem::Specification.new do |gem|
 
   gem.required_ruby_version = ">= 2.7.0"
 
-  gem.add_dependency 'mutex_m'
-
   gem.add_development_dependency 'rake'
   gem.add_development_dependency 'bump'
   gem.add_development_dependency 'activesupport'