poly: more parsing quirks

[chsievers]

• The outermost expression can't be a let expression:

Poly> let x=0 in x
"<stdin>" (line 1, column 11):
unexpected reserved word "in"
expecting letter or digit

• Unlike let declarations, let expressions don't allow function definitions with arguments:

Poly> (let f x = x in f 0)
"<stdin>" (line 1, column 8):
unexpected "x"
expecting "="

• let rec expressions aren't recursive:

Poly> (let rec x = x in x)
Not in scope: "x"

• Since fix is a special syntactic form that has to be applied to an expression, I expected g fix x not to parse. Instead, it is parsed as g (fix x).