Build a fallback ToUnicode map for simple fonts (issue 8229)

In some fonts, the included `ToUnicode` data is incomplete causing text-selection to not work properly. For simple fonts that contain encoding data, we can manually build a `ToUnicode` map to attempt to improve things.

Please note that since we're currently using the `ToUnicode` data during glyph mapping, in an attempt to avoid rendering regressions, I purposely didn't want to amend to original `ToUnicode` data for this text-selection edge-case.
Instead, I opted for the current solution, which will (hopefully) give slightly better text-extraction results in PDF file with incomplete `ToUnicode` data.

According to the PDF specification, see [section 9.10.2](http://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/PDF32000_2008.pdf#G8.1873172):

> A conforming reader can use these methods, in the priority given, to map a character code to a Unicode value.
> ...

Reading that paragraph literally, it doesn't seem too unreasonable to use *different* methods for different charcodes.

Fixes 8229.

test/pdfs/.gitignore

@@ -53,6 +53,7 @@
 !issue8061.pdf
 !issue8088.pdf
 !issue8125.pdf
+!issue8229.pdf
 !issue8372.pdf
 !issue8424.pdf
 !issue8480.pdf

test/test_manifest.json

@@ -1438,6 +1438,13 @@
        "link": false,
        "type": "text"
     },
+    {  "id": "issue8229",
+       "file": "pdfs/issue8229.pdf",
+       "md5": "a729f663782e87ebc1efad0755ebf6a5",
+       "rounds": 1,
+       "link": false,
+       "type": "text"
+    },
     {  "id": "ShowText-ShadingPattern",
        "file": "pdfs/ShowText-ShadingPattern.pdf",
        "md5": "fe683725db037ffe19d390969610a652",