Context bindings - Type Safety / IntelliSense

[superkhau]

See https://github.com/strongloop/loopback-next/pull/61/files#r102336527

We need to make it easy to know the input and output of the ctx's APIs. Here are some examples

// Hard to remember the string, easy to introduce typos
ctx.bind('someLongStringHere').to(SomeType);

// Does MyConcreteType implement MyInterface ???
// would be nice if the compiler checked this for me
ctx.bind('MyInterface').to(MyConcreteType);

// when I refactor code - I don't want to have to go find/replace all instances of 'someString'
ctx.bind('someString').to(SomeValue);

[bajtos]

Beware: symbols don't work across module boundaries. For example, if a 3rd party component wants to retrieve a context value using a symbol defined in a loopback module, and it happens that this loopback module exists twice in app's node_modules (once at top-level, second time as a dependency of the 3rd party component), then the lookup will fail. See my work in #54 for a proof.

[ritch]

I'm going to reword the issue - so we can focus on the problem instead of the implementation.

[dhmlau]

Once #1169 has landed, this issue will be resolved.