[sync] crypto/x509: rework path building

[emmansun]

golang/go@65153e4

This change does four things:

• removes the chain cache
• during path building, equality is determined by checking if the
  subjects and public keys match, rather than checking if the entire
  certificates are equal
• enforces EKU suitability during path building
• enforces name constraints on intermediates and roots which have
  SANs during path building

The chain cache is removed as it was causing duplicate chains to be
returned, in some cases shadowing better, shorter chains if a longer
chain was found first.

Checking equality using the subjects and public keys, rather than the
entire certificates, allows the path builder to ignore chains which
contain cross-signature loops.

EKU checking is done during path building, as the previous behavior
of only checking EKUs once the path had been built caused the path
builder to incorrectly ignore valid paths when it encountered a path
which would later be ruled invalid because of unacceptable EKU usage.

Name constraints are applied uniformly across all certificates, not
just leaves, in order to be more consistent.