Improve the performance `pdPageExtractText`

[sambitdash]

pdPageExtractText API is one the core APIs of PDFIO. However, smaller large number of allocations make it a bit slower. This code needs to be refactored to ensure the text extraction speeds are improved further.

Any inputs, proposals and PRs in this direction will be highly appreciated.

[sambitdash]

#58 addresses few of the issues.

[sambitdash]

#105 fixes a few cases.