Add state assertions to redbean Lua APIs

Many of the API functions provided by redbean are only appropriate to
call in certain contexts, such as request handling or .init.lua, etc.
For example, Fetch can't be called from the global scope of .init.lua
because SSL hasn't been configured yet. Earlier if this happened then
redbean would crash, which was confusing. What we'll do now is show a
friendly error message. See #97

This change also undocuments redbean ssl compression support since it
seems to be causing a flake in the testing infrastructure.

tool/net/demo/printpayload.lua

@@ -1,3 +1,12 @@
-SetStatus(200)
-SetHeader("Content-Type", GetHeader("Content-Type") or "text/plain")
-Write(GetPayload())
+local function main()
+   if GetMethod() ~= 'POST' and GetMethod() ~= 'PUT' then
+      ServeError(405)
+      SetHeader('Allow', 'POST, PUT')
+      return
+   end
+   SetStatus(200)
+   SetHeader("Content-Type", GetHeader("Content-Type") or "text/plain")
+   Write(GetPayload())
+end
+
+main()