Optimize public key serialization.

This change removes the internal pad function in favor a more opimized
paddedAppend function.  Unlike pad, which would always alloate a new
slice of the desired size and copy the bytes into it, paddedAppend
only appends the leading padding when necesary, and uses the builtin
append to copy the remaining source bytes.  pad was also used in
combination with another call to the builtin copy func to copy into a
zeroed byte slice.  As the slice is now created using make with an
initial length of zero, this copy can also be removed.

As confirmed by poking the bytes with the unsafe package, gc does not
zero array elements between the len and cap when allocating slices
with make().  In combination with the paddedAppend func, this results
in only a single copy of each byte, with no unnecssary zeroing, when
creating the serialized pubkeys.  This has not been tested with other
Go compilers (namely, gccgo and llgo), but the new behavior is still
functionally correct regardless of compiler optimizations.

The TestPad function has been removed as the pad func it tested has
likewise been removed.

ok @davecgh