apl_entry: Combine ENTRY_*_INT/FLOAT symbols

Doesn't matter whether the score or max values are being described,
both share two possible types: float and int
apl_entry: Stack-allocate the entry label

No reason to deal with a bunch of memory allocations for such small
chunks. 64 bytes is a good chunk size, and a power of 2. Shouldn't
leave much wasted space in the heap
apl_entry: Add apl_newentry method

Creates a new gradebook entry with a given label. Starting score/max are
both zero
Add apl_entry sanity test