Added repl and some documentation. Improved error messages. Removed dead code.

* Removed same_variant in parse::parsetree
* Added SExp::multistep (for use of the repl)

Improved error messages:
* Added parenthesis around types
* Changed how errors propagate inferring generics:
  added the error variant ArgumentsDontMatchGeneric,
  implemented the displaying of it, added tests for
  it.
* Had to change some tests to match for the new changes

1 files changed, 84 insertions, 0 deletions

diff --git a/TUTORIAL.md b/TUTORIAL.md
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+
+
+Tutorial
+========
+
+This tutorial consists of a language tour
+and a set of exercises, in the order of
+mentioning.
+
+
+Tour
+----
+
+
+**Arithmetic operations**
+
+Addition, multiplication, subtraction and division
+are supported with +, *, - and / respectively.
+```console
+> (+ 1 1)
+2 : Int
+> (* 2 3)
+6 : Int
+> (- 5 4)
+1 : Int
+> (/ 4 2)
+2 : Int
+```
+
+Decimals are truncated in division:
+```console
+> (/ 5 2)
+2 : Int
+```
+
+
+**Lists**
+
+Lists in myslip correspond to what is known as tuples in many
+other programming languages. This difference exists because
+myslip is a list processor, and not using this underlying
+construct of the language would be odd.
+
+In principle,
+```myslip
+(1 2 3 4 5)
+```
+is a valid list, but it is evaluated by the interpreter,
+which assumes that the first term, `1`, is an operator.
+That's why constructing a list requires the operator
+`quote`:
+```console
+> quote
+quote : (T -> T)
+> (quote 1 2 3 4 5)
+(quote 1 2 3 4 5) : (Int Int Int Int Int)
+```
+
+As hinted above, `quote` is typewise the identity function.
+Behaviorally it does though simplify sub-expressions.
+```console
+> (quote (+ 1 1) (+ 2 2))
+(quote 2 4) : (Int Int)
+```
+
+The elements of a list can of course be of different types:
+```console
+> (quote + 0 (quote 1 2))
+(quote + 0 (quote 1 2)) : (((Int Int) -> Int) Int (Int Int))
+```
+
+TODO: List destructuring
+
+
+**Understanding error messages**
+TODO: div zero
+TODO: unclosed parenthesis
+TODO: type errors
+
+
+Exercises
+---------
+
+TODO