Pattern guards for functions and switch statements (#339)

* Add case guards to parser

* Add pattern guards to infer types and fcode generation

* Add functions guards

* Add test for patterns guards

* Update docs

* Update CHANGELOG.md

* Remove stateful context from Env for guards

* Elaborate on guards

* Add failing test for stateful pattern guards

* Implement multiple guards

* Fix tests

* Disable aevm related tests

* Split the sentence before if and otherwise

* Fix type in docs

* Implement multiple exprs in the same guard

* Fix pretty printing

* Change tests to include multiple guards

* Add test for non-boolean guards

* Desugar clauses with guards

* Fix incomplete patterns bug

* Fix docs

* Compile to icode when no guards are used

* Revert "Disable aevm related tests"

This reverts commit e828099bd97dffe11438f2e48f3a92ce3641e85b.

docs/sophia_features.md

@@ -121,7 +121,7 @@ contract IntHolder =
   type state = int
   entrypoint init(x) = x
   entrypoint get() = state
-  
+
 main contract IntHolderFactory =
   stateful entrypoint new(x : int) : IntHolder =
     let ih = Chain.create(x) : IntHolder
@@ -471,6 +471,8 @@ function
   get_left(Both(x, _)) = Some(x)
 ```
 
+*NOTE: Data types cannot currently be recursive.*
+
 Sophia also supports the assignment of patterns to variables:
 ```sophia
 function f(x) = switch(x)
@@ -482,7 +484,28 @@ function g(p : int * option(int)) : int =
   b
 ```
 
-*NOTE: Data types cannot currently be recursive.*
+Guards are boolean expressions that can be used on patterns in both switch
+statements and functions definitions. If a guard expression evaluates to
+`true`, then the corresponding body will be used. Otherwise, the next pattern
+will be checked:
+
+```sophia
+function get_left_if_positive(x : one_or_both(int, 'b)) : option(int) =
+  switch(x)
+    Left(x)    | x > 0 => Some(x)
+    Both(x, _) | x > 0 => Some(x)
+    _                  => None
+```
+
+```sophia
+function
+  get_left_if_positive : one_or_both(int, 'b) => option(int)
+  get_left_if_positive(Left(x))    | x > 0 = Some(x)
+  get_left_if_positive(Both(x, _)) | x > 0 = Some(x)
+  get_left_if_positive(_)                  = None
+```
+
+Guards cannot be stateful even when used inside a stateful function.
 
 ## Lists
 
@@ -851,4 +874,4 @@ Some chain operations (`Oracle.<operation>` and `AENS.<operation>`) have an
 optional delegation signature. This is typically used when a user/accounts
 would like to allow a contract to act on it's behalf. The exact data to be
 signed varies for the different operations, but in all cases you should prepend
-the signature data with the `network_id` (`ae_mainnet` for the æternity mainnet, etc.).
+the signature data with the `network_id` (`ae_mainnet` for the æternity mainnet, etc.).