Updates to use rust 1.85. Doesn't move to the 2024 edition, as that's a
fair bit more involved.
A nice side benefit is that the new rustc version seems to lead to a
slight reduction in binary size (at least on mac):
```
FILE SIZE
--------------
+4.3% +102Ki __DATA_CONST,__const
[NEW] +69.3Ki __TEXT,__literals
[NEW] +68.5Ki Rebase Info
+5.0% +39.9Ki __TEXT,__unwind_info
+57% +8.85Ki [__TEXT]
[NEW] +8.59Ki Lazy Binding Info
[NEW] +5.16Ki __TEXT,__stub_helper
[NEW] +3.58Ki Export Info
[NEW] +3.42Ki __DATA,__la_symbol_ptr
-0.1% -726 [12 Others]
-21.4% -3.10Ki [__DATA_CONST]
-95.8% -3.39Ki __DATA_CONST,__got
-20.9% -3.43Ki [__DATA]
-0.5% -4.52Ki Code Signature
-100.0% -11.6Ki [__LINKEDIT]
-1.0% -43.5Ki Symbol Table
-1.6% -44.0Ki __TEXT,__gcc_except_tab
-0.2% -48.1Ki __TEXT,__const
-3.3% -78.6Ki __TEXT,__eh_frame
-0.7% -320Ki __TEXT,__text
-1.5% -334Ki String Table
-0.5% -586Ki TOTAL
```
