Import optimization

[bavshin-f5]

Dmitry has noted that the amount of imports required for even the smallest module example is overwhelming.

We can approach this from several directions:

1. Expose a minimal sufficient set of symbols via ngx::core::prelude::* and ngx::http::prelude::*
2. Use enums and bitflags as much as possible.
   E.g. wrapping NGX_CONF_... with bitflags would save a few imports and remove noisy as ngx_uint_t casts.
3. Reduce boilerplate code. For example,

   #[no_mangle]
   #[used]
   pub static mut ngx_http_example_module: ngx_module_t = ngx_module_t {
     ctx: std::ptr::addr_of!(NGX_HTTP_EXAMPLE_MODULE_CTX) as _,
     // Safety: nginx does not modify module.commands.
     commands: NGX_HTTP_EXAMPLE_COMMANDS.as_ptr().cast_mut(),
     type_: NGX_HTTP_MODULE as _
   
     .. NGX_RS_MODULE_V1
   }

   would remove a couple of imports that are never necessary in the code and reduce amount of copy-paste needed to get started.