Rename client methods

[pngwn]

• I have searched to see if a similar issue already exists.

Rename the following class methods:

• fetch_implementation -> fetch
  • is is common for fetch 'replacements' to be referenced as this.fetch fetch correctly describes this behaviour (even though it is also the name of the platform builtin). This language is universal
• event_source -> stream
  • the fact we use an event source here is an implementation detail, so this name is leaky. There is no requirement for this to be an instance of EventSource and indeed it isn't in the client.

Aside form these methods being more descriptive they are also nicer to work with. They are now exposed to component directly (including custom components), so this is public API and not internal.