reflect: make struct field offset a varint

[aykevl]

#3656 added struct sizes and field offsets. This adds around 0.36% on average to the binary size according to my test, although the increase is highly variable (and sometimes negative).

I suspect most of the size increase comes from the struct field offsets. One way to reduce the overhead is by not storing the offset as a uint32, but rather as a variable width integer in the data byte array. I believe that will save 3 bytes per struct field in the common case.