DEFINE_SIZE_MIN is to be used only in rare cases. For fixed-size structs
we want DEFINE_SIZE_STATIC.

We always just used "unsigned int" for counter values. There's
no use for uint16_t outside of a struct.

Also, no need for explict casting where implicit does.

Ops, this was done for testing

Maybe we should make DEFINE_* make structs uncopyable.

https://github.com/harfbuzz/harfbuzz/pull/821#issuecomment-368116538

[subset] Caclculate head table checksum adjustment

Test still fails, because we do not serialize tables in the same
order that fonttools subsetter does.

[subset] Fix several memory leaks in subsetting.