Pre-allocate memory for scalar functions more effectively

For most of our scalar functions we do something like this:

```rust
let mut builder = BinaryBuilder::with_capacity(
executor.num_iterations(),
WKB_MIN_PROBABLE_BYTES * executor.num_iterations(),
);
```

A lot of them, however, can do a better job pre-allocating the output based on previously available information (or potentially a cheap iteration over the values). For example, ST_Boundary output is usually similar to that of the input, and ST_Reverse is always exactly the same as the input.

It's possible this won't make much of a difference but there are at least some functions where this is trivial.

Search the scalar-function implementations for BinaryBuilder::with_capacity, then compare ST_Boundary and ST_Reverse with their input values. Identify functions where output size is available or cheap to estimate, and consider the work done when those functions pre-allocate more accurately without changing results.

Written by the indexing model from the issue text.