adjust runtime page size APIs

* fix merge conflicts
* rename the declarations
* reword documentation
* extract FixedBufferAllocator to separate file
* take advantage of locals
* remove the assertion about max alignment in Allocator API, leaving it
  Allocator implementation defined
* fix non-inline function call in start logic

The GeneralPurposeAllocator implementation is totally broken because it
uses global state but I didn't address that in this commit.

lib/std/Thread.zig

@@ -1155,7 +1155,7 @@ const LinuxThreadImpl = struct {
         completion: Completion = Completion.init(.running),
         child_tid: std.atomic.Value(i32) = std.atomic.Value(i32).init(1),
         parent_tid: i32 = undefined,
-        mapped: []align(std.heap.min_page_size) u8,
+        mapped: []align(std.heap.page_size_min) u8,
 
         /// Calls `munmap(mapped.ptr, mapped.len)` then `exit(1)` without touching the stack (which lives in `mapped.ptr`).
         /// Ported over from musl libc's pthread detached implementation: