Simplify the syntax of row types

[yannham]

Is your feature request related to a problem? Please describe.
The syntax of record and enum types is verbose. A few examples:

{ {| a: Num |} }
forall r. { {| a: Num, b: Str | r} }
< (| tag1 |) >
forall r. < (| tag1, tag2 | r) >

Describe the solution you'd like
Get rid of the unnecessary inner delimiters {| |} and (| |). Only use the pipe to separate fields from the polymorphic tail. This is for example the syntax of records in PureScript, although they use :: for typing, as in Haskell.

The examples would become:

{a: Num}
forall r. {a: Num, b: Str | r}
<tag1>
forall r. <tag1, tag2 | r>