Add literal string constants (#19)

In addition, for the sake of Haskell code generation, this PR adds static typing
  with Statix.

  Resolves #5.

Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: glen/fostr#19
Co-Authored-By: Glen Whitney <glen@nobody@nowhere.net>
Co-Committed-By: Glen Whitney <glen@nobody@nowhere.net>

tests/basic.spt

@@ -1,6 +1,16 @@
 module basic
 language fostr
 
+test hw1_type [[
+[[stream]] << [['Hello,	world! ']] << [[3+2]] << ' times.'
+]]
+run get-type on #1 to STREAM()
+run get-type on #2 to STRING()
+run get-type on #3 to INT()
+run get-type to STREAM()
+/** writes
+Hello,	world! 5 times.**/
+
 /** md
 Title: A whirlwind tour of fostr
 

tests/emit_thrice.fos

@@ -1,4 +1,4 @@
-  stream << 72 + 87
+  stream << 'Some numbers: '
   stream << 88
     + 96
   99 + 12 >>

tests/hw.fos

@@ -0,0 +1 @@
+stream << 'Hello,	world!'