Certificate chain verification

[nick-mobilecoin]

• Add logic to decode and verify certificate #49
• Add logic to verify and get signing key from chain #70
  • Add initial CRL logic to cert chain verification #71
• Add verification of certificate time #51
• What do we do about alloc? Updated Add logic to decode and verify certificate #49 to use an alloc feature
• Move x509 cert logic to sub-directory #68
• Build a valid certificate chain from a collection of certificates, https://www.rfc-editor.org/rfc/rfc4158
• Need to consider robust test data like https://csrc.nist.gov/projects/pki-testing #79
• Need to define/determine how much of https://www.rfc-editor.org/rfc/rfc5280 we're going to support
• ignore bidi code points as specified in https://www.rfc-editor.org/rfc/rfc4518#section-2.5
• fail on prohibit code points as specified in https://www.rfc-editor.org/rfc/rfc4518#section-2.5 (Note that these points specified are from unicode 3.2)