Use const char* with binary encoded values

When encoding values, there is no need to modify the encoded data when
writing it out. So mark the pointers used to reference the encoded data
as `const`. While there is nothing happening here that should cause
issues, this will help further safeguard developers making changes here
and clarify the intent.

Author: jakirkham

numcodecs/vlen.pyx

@@ -80,7 +80,7 @@ class VLenUTF8(Codec):
             ndarray[object, ndim=1] input_values
             object[:] encoded_values
             int[:] encoded_lengths
-            char* encv
+            const char* encv
             bytes b
             bytearray out
             char* data
@@ -214,7 +214,7 @@ class VLenBytes(Codec):
             object[:] values
             object[:] normed_values
             int[:] lengths
-            char* encv
+            const char* encv
             object o
             bytes b
             bytearray out