In node v10.0, build failed

Hi, is there way to avoid this problem?

```
gyp info spawn args [ 'BUILDTYPE=Release', '-C', 'build' ]
CC(target) Release/obj.target/bcoin-native/src/poly1305-donna/poly1305-donna.o
CC(target) Release/obj.target/bcoin-native/src/chacha20-simple/chacha20_simple.o
CC(target) Release/obj.target/bcoin-native/src/scrypt/insecure_memzero.o
CC(target) Release/obj.target/bcoin-native/src/scrypt/sha256.o
CC(target) Release/obj.target/bcoin-native/src/scrypt/crypto_scrypt.o
CXX(target) Release/obj.target/bcoin-native/src/chacha20.o
CXX(target) Release/obj.target/bcoin-native/src/poly1305.o
CXX(target) Release/obj.target/bcoin-native/src/digest.o
../src/digest.cc:28:14: error: variable has incomplete type 'EVP_MD_CTX' (aka 'evp_md_ctx_st')
EVP_MD_CTX mdctx;
^
/Users/xxxx/.node-gyp/10.0.0/include/node/openssl/ossl_typ.h:92:16: note: forward declaration of 'evp_md_ctx_st'
typedef struct evp_md_ctx_st EVP_MD_CTX;
^
../src/digest.cc:58:12: error: variable has incomplete type 'HMAC_CTX' (aka 'hmac_ctx_st')
HMAC_CTX hmctx;
^
/Users/xxxx/.node-gyp/10.0.0/include/node/openssl/ossl_typ.h:102:16: note: forward declaration of 'hmac_ctx_st'
typedef struct hmac_ctx_st HMAC_CTX;
^
2 errors generated.
make: *** [Release/obj.target/bcoin-native/src/digest.o] Error 1
```

The compiler points to src/digest.cc; start there and reproduce the native build with Node v10.0. Investigate the incomplete EVP_MD_CTX and HMAC_CTX errors, then verify that the native module builds without these failures.

Written by the indexing model from the issue text.