sourcewalk (etc) drops all but the first character in replacements

This is hard to explain.
here is a demo

I want to remove the second argument of a 3 argument function.

```
julia> using MacroTools: textwalk

julia> code = """
test_scalar(cos, x -> -sin(x), x)
test_scalar(tan, x -> 1 + tan(x)^2, x)
test_scalar(sec, x -> sec(x) * tan(x), x)
test_scalar(csc, x -> -csc(x) * cot(x), x)
""";

julia> function twoarg(expr)
@capture(expr, t_(f_, d_, x_)) && t==:test_scalar && return :(test_scalar($(f), $x))
return expr
end
twoarg (generic function with 1 method)

julia> textwalk(twoarg, code) |> println
test_scalar(c, x)
test_scalar(t, x)
test_scalar(s, x)
test_scalar(c, x)
```

Start by reproducing the Julia textwalk example from the issue and then inspect the textwalk/sourcewalk replacement path. Done means the transformation removes the second argument while preserving each complete function name, rather than reducing names to their first character.

Written by the indexing model from the issue text.