[Feature request] Feature to ensure a certain piece of data is within aligned boundary

This is not really easy to implement, but I though I'd post it before I forget again so it's on the record as a feature request.

The feature requested is the ability to make sure that a certain piece of data (for example a table or a short piece of data to be copied) doesn't cross a page boundary. This would allow optimizations in many cases where 8 bit math can be used without checking overflow, and without the need for hard alignment to a page boundary, which imposes more restrictions on the alignment than necessary. Consider something like:
```
; A contains an index to the table.
ld D, HIGH(some_table)
add LOW(some_table) ; This is ok because we can assume the table won't cross a page boundary.
ld E,A
ld A, [DE]

; ....
BALIGN 8
some_table:
db xx, xx, xx, xx
ENDBALIGN
```
Or:
```
small_copy::
ld A,[DE]
ld [HL+],A
inc E ; This is ok because we can assume DE will not cross a page boundary.
dec B
jr nz,small_copy
ret
; ....
ld DE,some_data
call small_copy

; ....
BALIGN 8
some_data:
db xx, xx, xx, xx
ENDBALIGN

```

Start by reviewing the existing BALIGN and ENDBALIGN handling in the assembler and how data placement is represented. Trace how alignment constraints are validated, then determine where a no-page-crossing constraint could fit. Done means the requested data can be constrained not to cross a page boundary without requiring hard alignment, with coverage for the examples described here.

Written by the indexing model from the issue text.