Support NRT (nullable reference types / nullness annotation)

[jwosty]

Attempting to build a fresh SAFE stack template after upgrading everything to latest and turning on <Nullable>enable</Nullable> results in some compile errors from this library:

.\output\fable_modules\Fable.Elmish.React.5.0.1\common.fs(11,62): (11,67) error FSHARP: A type parameter is missing a constraint 'when 'P: not null' (code 1)
.\output\fable_modules\Fable.Elmish.React.5.0.1\common.fs(11,62): (11,67) error FSHARP: Type constraint mismatch. The type     ''P'    is not compatible with type    'obj' (code 193)
.\output\fable_modules\Fable.Remoting.MsgPack.1.25.0\Write.fs(792,21): (792,22) error FSHARP: A type parameter is missing a constraint 'when 'T: not null' (code 1)
.\output\fable_modules\Fable.SimpleJson.3.24.0\SimpleJson.fs(88,32): (88,36) error FSHARP: The constraints 'null' and 'not null' are inconsistent (code 43)
.\output\fable_modules\Feliz.3.1.1\React\React.fs(42,47): (42,51) error FSHARP: The constraints 'null' and 'not null' are inconsistent (code 43)
.\output\fable_modules\Feliz.3.1.1\React\React.fs(45,44): (45,48) error FSHARP: The constraints 'null' and 'not null' are inconsistent (code 43)
.\output\fable_modules\Feliz.3.1.1\React\React.fs(56,47): (56,51) error FSHARP: The constraints 'null' and 'not null' are inconsistent (code 43)
.\output\fable_modules\Feliz.3.1.1\React\React.fs(58,44): (58,48) error FSHARP: The constraints 'null' and 'not null' are inconsistent (code 43)
.\output\fable_modules\Feliz.3.1.1\React\React.fs(70,45): (70,49) error FSHARP: The constraints 'null' and 'not null' are inconsistent (code 43)
.\output\fable_modules\Feliz.3.1.1\React\React.fs(73,42): (73,46) error FSHARP: The constraints 'null' and 'not null' are inconsistent (code 43)
.\output\fable_modules\Feliz.3.1.1\React\React.fs(419,26): (419,36) error FSHARP: A type parameter is missing a constraint 'when 'props: not null'. See also C:/Code/AdBridge/src/AdBridge.Client/output/fable_modules/Feliz.3.1.1/React/React.fs(419,25)-(419,35). (code 43)
.\output\fable_modules\Fable.SimpleJson.3.24.0\Json.Converter.fs(682,41): (682,70) error FABLE: System.Uri.ToString is not supported by Fable

Project builds fine with NRT disabled, so I'm assuming it's just some annotation issues.

Related: fable-compiler/Fable#3887