Support and keyword in computation expression

[nojaf]

Issue created from fantomas-ui

Support https://github.com/fsharp/fslang-design/blob/master/RFCs/FS-1063-support-letbang-andbang-for-applicative-functors.md

Code

// Reads the values of x, y and z concurrently, then applies f to them
parallel {
    let! x = slowRequestX()
    and! y = slowRequestY()
    and! z = slowRequestZ()
    return f x y z
}

Result

// Reads the values of x, y and z concurrently, then applies f to them
``parallel`` {
    let! x = slowRequestX()
    return f x y z }

Options

Fantomas Next - 3.2.0-2/10/2020

Name	Value
IndentOnTryWith	false
IndentSpaceNum	4
KeepNewlineAfter	false
MaxIfThenElseShortWidth	40
PageWidth	120
ReorderOpenDeclaration	false
SemicolonAtEndOfLine	false
SpaceAfterComma	true
SpaceAfterSemicolon	true
SpaceAroundDelimiter	true
SpaceBeforeArgument	true
SpaceBeforeColon	false
StrictMode	false

[nojaf]

Other examples:

// Outputs a + b, which is recomputed every time foo or bar outputs a new value,
// avoiding any unnecessary resubscriptions
observable {
    let! a = foo
    and! b = bar
    return a + b
}