* Reduce Var instance size by moving the diagnostic message string to a
hash map for the instances that really have it (there are actually very
few such instances).
* Move common members from Var subclasses to the base class and
represent them more compactly.
* Remove RuleVar wrapper class, it is an extra object per rule-specific
variable.
* Rename Expr class to ValueList.
* Use additional constructors and convenience factory methods in Value
and its subclasses to avoid creating ValueList with a single element and
then compacting it.
* Use method overloading instead of having NewExpr2/NewExpr3, move them
for the global namespace to be factory methods of the Value class.

Change-Id: I62a87c5d8309b21265e904c0aeb9b3e094c9024a

Test case:

$ cat Makefile.recursive
.PHONY: default
default:
        +echo DONE

$ ckati --warn --gen_all_targets --regen --ninja -f Makefile.recursive
Makefile.recursive was modified, regenerating...
$ fgrep DONE build.ninja
 command = /bin/sh -c "+echo DONE"

$ ninja
[1/1] build default
FAILED: default
/bin/sh -c "+echo DONE"
/bin/sh: + : invalid option
Usage:  /bin/sh [GNU long option] [option] ...
...

Fixes https://github.com/google/kati/issues/57

Change-Id: Ic40d71ff2b168158661eb2eac638adaeadaec1fe

This is for issue #40.

So we will be able to use the command line parser for sub-makes.