keep empty lines

[s-trooper]

please keep empty lines:

i.e for nested let's:

let f1 = 
    let f2 x = 
        x + 1
    let f3 y = 
        y + 1
    x + y

vs

let f1 = 
    let f2 x = 
        x + 1

    let f3 y = 
        y + 1

    x + y

or 2 empty lines for top level let's like python

[dungpa]

Steps to hand this issue:

• Recognize a list of LetOrUse patterns.
• Split them into one-liner and multiline chunks.
• Process those similar to what we did with let bindings, member declarations and signatures.

[dungpa]

@s-trooper The fix has been available in Fantomas extension v0.7.0. Thank you for reporting this.