In the original BoringCrypto port, ecdsa and rsa's public and private
keys added a 'boring unsafe.Pointer' field to cache the BoringCrypto
form of the key. This led to problems with code that “knew” the layout
of those structs and in particular that they had no unexported fields.
In response, as an awful kludge, I changed the compiler to pretend
that field did not exist when laying out reflect data. Because we want
to merge BoringCrypto in the main tree, we need a different solution.
The different solution is this CL's boring.Cache, which is a
concurrent, GC-aware map from unsafe.Pointer to unsafe.Pointer (if
generics were farther along we could use them nicely here, but I am
afraid of breaking tools that aren't ready to see generics in the
standard library yet).
More complex approaches are possible, but a simple, fixed-size hash
table is easy to make concurrent and should be fine.