dmd verus ldmd2 -lib option

[Dan Olson]

I noticed that dmd -lib breaks up a module into separate .o's when
building a static library.  This is could help make smaller executables
(I am thinking smart phones) when linking with phobos because it will
only pull in to .o's needed to satisfy what is referenced.

ldmd2 -lib doesn't split into multiple .o's like dmd.  Does it seem like
a good idea to do the same (I am thinking so).

Example: file testlib.d gets split into four .o's
cat >testlib.d <<eof
void foo() {}
void bar() {}
void xyzzy() {}
eof
$ dmd -lib -oflibtest.a testlib.d
$ nm libtest.a

libtest.a(testlib.o):
0000000000000050 D _D7testlib12__ModuleInfoZ
0000000000000030 T _D7testlib15__unittest_failFiZv
0000000000000000 T _D7testlib7__arrayZ
0000000000000018 T _D7testlib8__assertFiZv
                 U __d_array_bounds
                 U __d_assertm
                 U __d_unittestm

libtest.a(testlib_1_144.o):
0000000000000000 S _D7testlib3fooFZv

libtest.a(testlib_2_135.o):
0000000000000000 S _D7testlib3barFZv

libtest.a(testlib_3_25e.o):
0000000000000000 S _D7testlib5xyzzyFZv