make implicit typecast explicit

make implicit typecast explicit

This came out of adding these compile options:
`-Werror -Wall -Wextra -Wshadow -Wconversion -Wno-deprecated-declarations -Wno-unused-parameter`

/src/monocypher.c
-u32  index     = lane * lane_size + (u32)ref;
+u32  index     = (u32)(lane * lane_size) + (u32)ref;