Pull out essence total map essence of MachineInfos into PerMachine

We'll eventually have many other data structure duplicated for each
build, host, and target machines. This sets up the infrastructure for
that.

mesonbuild/environment.py

@@ -17,7 +17,7 @@ import configparser, os, platform, re, sys, shlex, shutil, subprocess
 from . import coredata
 from .linkers import ArLinker, ArmarLinker, VisualStudioLinker, DLinker
 from . import mesonlib
-from .mesonlib import EnvironmentException, Popen_safe
+from .mesonlib import EnvironmentException, PerMachine, Popen_safe
 from . import mlog
 
 from . import compilers
@@ -1147,11 +1147,9 @@ class MachineInfo:
             literal['cpu'],
             literal['endian'])
 
-class MachineInfos:
+class MachineInfos(PerMachine):
     def __init__(self):
-        self.build = None
+        super().__init__(None, None, None)
-        self.host = None
-        self.target = None
 
     def default_missing(self):
         """Default host to buid and target to host.

mesonbuild/mesonlib.py

@@ -20,6 +20,8 @@ import stat
 import time
 import platform, subprocess, operator, os, shutil, re
 import collections
+from enum import Enum
+
 from mesonbuild import mlog
 
 have_fcntl = False
@@ -277,6 +279,53 @@ def classify_unity_sources(compilers, sources):
             compsrclist[comp].append(src)
     return compsrclist
 
+class OrderedEnum(Enum):
+    """
+    An Enum which additionally offers homogeneous ordered comparison.
+    """
+    def __ge__(self, other):
+        if self.__class__ is other.__class__:
+            return self.value >= other.value
+        return NotImplemented
+
+    def __gt__(self, other):
+        if self.__class__ is other.__class__:
+            return self.value > other.value
+        return NotImplemented
+
+    def __le__(self, other):
+        if self.__class__ is other.__class__:
+            return self.value <= other.value
+        return NotImplemented
+
+    def __lt__(self, other):
+        if self.__class__ is other.__class__:
+            return self.value < other.value
+        return NotImplemented
+
+MachineChoice = OrderedEnum('MachineChoice', ['BUILD', 'HOST', 'TARGET'])
+
+class PerMachine:
+    def __init__(self, build, host, target):
+        self.build = build,
+        self.host = host
+        self.target = target
+
+    def __getitem__(self, machine: MachineChoice):
+        return {
+            MachineChoice.BUILD:  self.build,
+            MachineChoice.HOST:   self.host,
+            MachineChoice.TARGET: self.target
+        }[machine]
+
+    def __setitem__(self, machine: MachineChoice, val):
+        key = {
+            MachineChoice.BUILD:  'build',
+            MachineChoice.HOST:   'host',
+            MachineChoice.TARGET: 'target'
+        }[machine]
+        setattr(self, key, val)
+
 def is_osx():
     return platform.system().lower() == 'darwin'