Value bounds checking should respect `[requires]` attributes.

Currently, integer bounds checking ignores `[requires]`, which means that `$max_size_in_bytes` and related values can be excessively large in some cases; e.g.:

```
struct Foo:
0 [+4] UInt size
[requires: this <= 16]
4 [+4] UInt message_type
8 [+size] UInt:8[] payload
# $max_size_in_bytes == 0xffff_ffff
# The real max size is 8 + 16 == 24.
```

The `_compute_constraints_of_field_reference` function in `compiler/front_end/expression_bounds.py` would need to be augmented to look for `[requires]` attributes and parse them for inferrable constraints (e.g., `X && this <= 4`, where `X` is any expression, puts an upper bound of `4` on the value). There is some similar expression analysis in `write_inference.py`, though the analysis there does not do what would be needed for bounds inference.