TOML: Improve type-stability of BigInt/UInt support (#53955)
From a type-stability perspective, this restores a lot of our behavior
before #47903.
As it turns out, 10 of the 11 uses of `parse_int` (now called
`parse_integer`) introduced in that PR are unnecessary since the TOML
format already requires the parsed value to be within a very limited
range.
Note that this change does not actually revert any functionality (in
contrast to #49576)
