Allocate string types directly.

Allocate and free ASN.1 string types directly instead of going through
the ASN.1 item code.

(Imported from upstream's 3d6aa6d441fe8124d247dffee5c68c2e5efd8258.)

Change-Id: I617283e67071a792f219ed08f19078afc223e2f5
Reviewed-on: https://boringssl-review.googlesource.com/4041
Reviewed-by: Adam Langley <agl@google.com>