Implement interpolated string

[andylokandy]

This PR implements both #1027 and #555, basing on #1034.

For example, the listing below equals to "Hello Idris2!":

let idris = "Idris" in 
  "Hello \{idris ++ show 2}!"

For more details see #1027.

[andrevidela]

we can probably close #1034 and focus on this PR

[andylokandy]

@andrevidela Isn't it too large as a whole?

[buzden]

we can probably close #1034 and focus on this PR

It seems that #1034 has its own separate value disregarding raw strings.

[andylokandy]

@gallais @andrevidela @ohad I've implemented the interpolated string and I reused this PR (originally for raw strings) for it since they are quite related IMO. 😄

[gallais]

One last question