checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

var/spack/repos/builtin/packages/ceed/package.py

@@ -18,10 +18,10 @@ class Ceed(Package):
     homepage = "https://ceed.exascaleproject.org"
 
     url  = 'file://' + os.path.dirname(__file__) + '/README.md'
-    sha1 = 'b2eefd95c09ba573f663a761b84811a2d7e39788'
+    sha256 = '418c4b6b6a098648e25befdc28cdf9c351ecea9deaa660c0d9a674f8c6917122'
 
-    version('2.0.0', sha1, expand=False)
-    version('1.0.0', sha1, expand=False)
+    version('2.0.0', sha256, expand=False)
+    version('1.0.0', sha256, expand=False)
 
     variant('cuda', default=False,
             description='Build MAGMA; enable CUDA support in libCEED and OCCA')