x/tools/gopls: hover: show fraction of wasted space in structs

[adonovan]

https://go.dev/cl/573076 (pending) augments gopls' hover information with the size and offset of each field when viewing a struct. A recent discussion about the fieldalignment analyzer (which is disabled due to high false positives) made me wonder whether any static analysis (as opposed to dynamic analysis using the heap profiler) had anything to offer. I think it might: we could compute the size of the struct as if the fields were in space-optimal order (largest to smallest), and if this is significantly smaller than the actual size, or falls into a different allocator size class), then we could report the waste fraction too.

[gopherbot]

Change https://go.dev/cl/575375 mentions this issue: gopls/internal/golang: Hover show wasted % of struct space