https://github.com/jdeng/goheif
Based on libde265 for HEVC decoding and some modifications to @bradfitz's HEIF parser.
Works for Linux/Mac OS X/Windows (with TDM-GCC).
All golang code is less than 2000 lines including the HEIF parser (the rest of the package is the included libde265 decoder C++ code). Likely the smallest self contained open source option on internet.