Don't parse in parallel

Parsing in parallel is leading to unexpected `EXC_BAD_ACCESS`es in our codebase.  By switching to `compactMap`, the crashes go away.

The specific crash is deep in the internals of `SwiftSyntax`, which leads me to believe that there's some internal global thread-unsafe state that's being shared bewteen parsing instances. Here's an example (abbreviated) stacktrace:
```
#0	0x00000001001209e4 in protocol witness for SyntaxProtocol._syntaxNode.getter in conformance Syntax ()
#1	0x0000000100121ec3 in SyntaxProtocol.raw.getter at /Users/eric_horacek/Library/Developer/Xcode/DerivedData/Sourcery-hibchsafsxkdcjaxxaylrbeyoydh/SourcePackages/checkouts/swift-syntax/Sources/SwiftSyntax/Syntax.swift:129
#2	0x0000000100b85c32 in SimpleTypeIdentifierSyntax.init(_:) at /Users/eric_horacek/Library/Developer/Xcode/DerivedData/Sourcery-hibchsafsxkdcjaxxaylrbeyoydh/SourcePackages/checkouts/swift-syntax/Sources/SwiftSyntax/gyb_generated/syntax_nodes/SyntaxTypeNodes.swift:68
#3	0x0000000100b89979 in protocol witness for SyntaxProtocol.init(_:) in conformance SimpleTypeIdentifierSyntax ()
#4	0x00000001001acab7 in TypeSyntax.as<τ_0_0>(_:) at /Users/eric_horacek/Library/Developer/Xcode/DerivedData/Sourcery-hibchsafsxkdcjaxxaylrbeyoydh/SourcePackages/checkouts/swift-syntax/Sources/SwiftSyntax/gyb_generated/SyntaxBaseNodes.swift:421
```

I was able to reproduce this in the debugger, but there was no information available with either TSAN or ASAN.

Fixes #1009.

I know that a lot of folks rely on the performance of parsing in parallel and it only seems to be crashing on our codebase, so perhaps we could put this behind an argument?

Sourcery/Sourcery.swift

@@ -38,9 +38,9 @@ public class Sourcery {
     // content annotated with file annotations per file path to write it to
     fileprivate var fileAnnotatedContent: [Path: [String]] = [:]
 
-    private (set) var numberOfFilesThatHadToBeParsed: Int32 = 0
+    private (set) var numberOfFilesThatHadToBeParsed = 0
     func incrementFileParsedCount() {
-        OSAtomicIncrement32(&numberOfFilesThatHadToBeParsed)
+        numberOfFilesThatHadToBeParsed += 1
     }
 
     /// Creates Sourcery processor
@@ -309,7 +309,7 @@ extension Sourcery {
             numberOfFilesThatHadToBeParsed = 0
 
             var lastError: Swift.Error?
-            let results = parserGenerator.parallelCompactMap { parser -> FileParserResult? in
+            let results = parserGenerator.compactMap { parser -> FileParserResult? in
                 do {
                     return try self.loadOrParse(parser: parser, cachesPath: cachesDir(sourcePath: from))
                 } catch {