Rewrite Cosmopolitan Ar

The build/bootstrap/ar.com program is now tinier. This change reduces
its size from 140kb to 53kb. Nothing was traded away. Cosmopolitan Ar
performance is now 2x better than llvm-ar largely thanks to using the
copy_file_range() system call. This change homebrews a new allocation
API that addresses the shortcomings of the C standard library design.
Using these new balloc() and reballoc() functions I managed to reduce
memory consumption so much that Cosmpolitan Ar should now use roughly
100x fewer bytes of peak resident memory compared to llvm-ar. Correct
behavior with better compatibility has been assured. Binary output is
now pretty much bit-identical to llvm-ar, as of this change. This can
and should be the living proof we need to show that a better world is
possible for software.

libc/elf/struct/rel.h

@@ -3,9 +3,51 @@
 #include "libc/elf/scalar.h"
 #if !(__ASSEMBLER__ + __LINKER__ + 0)
 
+/*
+ * ELF relocation.
+ *
+ * Relocations let us easily apply fixups to compiled object code. This
+ * data structure represents the contents of an `sh_type` w/ `SHT_REL`.
+ *
+ * @see Elf64_Rela
+ */
 typedef struct Elf64_Rel {
+
+  /*
+   * Location to be modified.
+   *
+   * If `e_type` is `ET_REL` then this is a section data byte offset.
+   *
+   * If `e_type` isn't `ET_REL` then this is a virtual address.
+   */
   Elf64_Addr r_offset;
-  Elf64_Xword r_info; /** @see ELF64_R_{SYM,SIZE,INFO} */
+
+  /*
+   * Relocation type and symbol.
+   *
+   * This value may be created using:
+   *
+   *     r_info = ELF64_R_INFO(sym, type);
+   *
+   * This value may be read using:
+   *
+   *     Elf64_Word sym = ELF64_R_SYM(r_info);
+   *     Elf64_Word type = ELF64_R_TYPE(r_info);
+   *
+   * Where `sym` is a symbol index, and `type` might be:
+   *
+   * - `R_X86_64_64`
+   * - `R_X86_64_PC32`
+   * - `R_X86_64_GOTPCRELX`
+   * - `R_AARCH64_ABS64`
+   *
+   * Each relocation type specifies a mathematical formula that's used
+   * to compute the appropriate value for the fixed-up object code. If
+   * if needs an addend, then this struct doesn't have one, but it can
+   * still be embedded by the compiler in the location to be modified.
+   */
+  Elf64_Xword r_info;
+
 } Elf64_Rel;
 
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */