C: Add blake3_hasher_init_derive_key_len

blake3_hasher_init_derive_key_len is an alternative version of
blake3_hasher_init_derive_key which takes the context and its
length as separate parameters, and not together as a C string.

The motivation for this addition is making it easier for
bindings to this C library to call this function without
having to first copy over the context bytes just to add
one 0x00 byte at the end.

Notice that contrary to blake3_hasher_init_derive_key,
blake3_hasher_init_derive_key_len allows the inclusion of a
0x00 byte in the context. Given the rules about context string
selection, this byte is unlikely to be used as part of a context
string. But if for some reason it is ever given, it will be
included in the context string and processed like any other
non-alphanumeric byte would. For compatibility with
blake3_hasher_init_derive_key, bindings should still check for
the absence of 0x00 bytes.

c/README.md

@@ -116,6 +116,18 @@ and application-specific. A good default format for the context string
 is `"[application] [commit timestamp] [purpose]"`, e.g., `"example.com
 2019-12-25 16:18:03 session tokens v1"`.
 
+```c
+void blake3_hasher_init_derive_key_len(
+  blake3_hasher *self,
+  const void *context,
+  size_t context_len);
+```
+
+Like `blake3_hasher_init_derive_key`, except the `context` and its 
+length are given as separate parameters, and not together as a C string.
+This can be more convenient than the C string version when writing 
+bindings to other languages.
+
 ```c
 void blake3_hasher_finalize_seek(
   const blake3_hasher *self,

c/blake3.c

@@ -367,10 +367,11 @@ void blake3_hasher_init_keyed(blake3_hasher *self,
   hasher_init_base(self, key_words, KEYED_HASH);
 }
 
-void blake3_hasher_init_derive_key(blake3_hasher *self, const char *context) {
+void blake3_hasher_init_derive_key_len(blake3_hasher *self, const void *context, 
+                                       size_t context_len) {
   blake3_hasher context_hasher;
   hasher_init_base(&context_hasher, IV, DERIVE_KEY_CONTEXT);
-  blake3_hasher_update(&context_hasher, context, strlen(context));
+  blake3_hasher_update(&context_hasher, context, context_len);
   uint8_t context_key[BLAKE3_KEY_LEN];
   blake3_hasher_finalize(&context_hasher, context_key, BLAKE3_KEY_LEN);
   uint32_t context_key_words[8];
@@ -378,6 +379,10 @@ void blake3_hasher_init_derive_key(blake3_hasher *self, const char *context) {
   hasher_init_base(self, context_key_words, DERIVE_KEY_MATERIAL);
 }
 
+void blake3_hasher_init_derive_key(blake3_hasher *self, const char *context) {
+  blake3_hasher_init_derive_key_len(self, context, strlen(context));
+}
+
 // As described in hasher_push_cv() below, we do "lazy merging", delaying
 // merges until right before the next CV is about to be added. This is
 // different from the reference implementation. Another difference is that we

c/blake3.h

@@ -42,6 +42,8 @@ void blake3_hasher_init(blake3_hasher *self);
 void blake3_hasher_init_keyed(blake3_hasher *self,
                               const uint8_t key[BLAKE3_KEY_LEN]);
 void blake3_hasher_init_derive_key(blake3_hasher *self, const char *context);
+void blake3_hasher_init_derive_key_len(blake3_hasher *self, const void *context, 
+                                       size_t context_len);
 void blake3_hasher_update(blake3_hasher *self, const void *input,
                           size_t input_len);
 void blake3_hasher_finalize(const blake3_hasher *self, uint8_t *out,