This makes it clearer that the helpers return a `Node` of any kind, and that the type is not important.
This also streamlines the API, as attribute helpers already return just `Node`.
The `Placer` interface was a weird interface that tried to abstract away nodes being elements or attributes, but it doesn't really make sense. Now `Nodes` just have a `NodeType`.
Previously, elements of kind void and empty elements generally would be rendered auto-closing (with a final `/` character in the start tag), which is allowed sometimes but arguably wrong. See https://dev.w3.org/html5/spec-LC/syntax.html#end-tags
This created problems with for example `textarea` and `script`, which cannot be auto-closing, or the browser renders it wrong.
Also clarified in the docs that this library outputs HTML5.
Fixes#42.
The Render function has been changed to take a `Writer` instead of returning a string. This makes it possible to generate documents without having the whole content in memory.
This also removes the `gomponents.Write` function, which is now redundant.
Furthermore, the `el.Document` function has been changed to only take one child, as multiple children never make sense for it. (It's not even a child, more a sibling.)
Just concatenating the strings is much faster.
Before:
```
make benchmark
go test -bench=.
goos: darwin
goarch: amd64
pkg: github.com/maragudk/gomponents
BenchmarkAttr/boolean_attributes-8 8194791 139 ns/op
BenchmarkAttr/name-value_attributes-8 5143292 229 ns/op
PASS
ok github.com/maragudk/gomponents 2.841s
```
After:
```
make benchmark
go test -bench=.
goos: darwin
goarch: amd64
pkg: github.com/maragudk/gomponents
BenchmarkAttr/boolean_attributes-8 16755404 67.0 ns/op
BenchmarkAttr/name-value_attributes-8 10208625 116 ns/op
PASS
ok github.com/maragudk/gomponents 2.702s
```
When implemented, the `Place` method of the `Placer` interface tells `Render` in `El` where to put a Node. This is relevant for helpers that want to be rendered like attributes, inside the parent element.
Fixes the bug where `attr.Classes` was rendered outside the element.
