math: Make function parentheses optional (#7877)

* math: Make function parentheses optional

It's a bit annoying to use parentheses here because that requires
quoting or escaping.

This allows the parens to be omitted, so

math sin pi

is the same as

math 'sin(pi)'

Function calls have the lowest precedence, so

math sin 2 + 6

is the same as

math 'sin(2 + 6)'

* Add more tests

* Add a note to the docs

* even moar docs

Moar docca

* moar tests

Call me Nikola Testla

tests/checks/math.fish

@@ -171,9 +171,7 @@ math "bitor(37 ^ 5, 255)"
 # CHECK: 69343999
 
 math 'log 16'
-# CHECKERR: math: Error: Missing opening parenthesis
-# CHECKERR: 'log 16'
-# CHECKERR:       ^
+# CHECK: 1.20412
 
 math 'log(16'
 # CHECKERR: math: Error: Missing closing parenthesis
@@ -195,3 +193,26 @@ echo $status
 
 math 'log2(8)'
 # CHECK: 3
+
+# same as sin(cos(2 x pi))
+math sin cos 2 x pi
+# CHECK: 0.841471
+# Inner function binds stronger, so this is interpreted as
+# pow(sin(3,5))
+
+math pow sin 3, 5
+# CHECKERR: math: Error: Too many arguments
+# CHECKERR: 'pow sin 3, 5'
+# CHECKERR: ^
+
+math sin pow 3, 5
+# CHECK: -0.890009
+
+math pow 2, cos -pi
+# CHECK: 0.5
+
+# pow(2*cos(-pi), 2)
+# i.e. 2^2
+# i.e. 4
+math pow 2 x cos'(-pi)', 2
+# CHECK: 4