I'm allowing incremental compilation of ZIR modules to be broken. This
is not a real use case of ZIR, and the feature requires a lot of code
duplication with incremental compilation of Zig AST (which works great).
The binary file abstraction changed its struct named "Decl" to
"TextBlock" and it now represents an allocated slice of memory in
the .text section. It has two new fields: prev and next, making it
a linked list node. This allows a TextBlock to find its neighbors.
The ElfFile struct now has free_list and last_text_block fields.
Doc comments for free_list are reproduced here:
A list of text blocks that have surplus capacity. This list can have false
positives, as functions grow and shrink over time, only sometimes being added
or removed from the freelist.
A text block has surplus capacity when its overcapacity value is greater than
minimum_text_block_size * alloc_num / alloc_den. That is, when it has so
much extra capacity, that we could fit a small new symbol in it, itself with
ideal_capacity or more.
Ideal capacity is defined by size * alloc_num / alloc_den.
Overcapacity is measured by actual_capacity - ideal_capacity. Note that
overcapacity can be negative. A simple way to have negative overcapacity is to
allocate a fresh text block, which will have ideal capacity, and then grow it
by 1 byte. It will then have -1 overcapacity.
The last_text_block keeps track of the end of the .text section.
Allocation, freeing, and resizing decls are all now more sophisticated,
and participate in the virtual address allocation scheme. There is no
longer the possibility for virtual address collisions.
