Fileseq

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* go/cpp: fix regex to handle parsing complex extensions with single character component ("file.1.a.ext")
* go/cpp: improve handling of ambiguous frame patterns in FindSequenceOnDisk ("100.1.ext")
* go: remove print statement from FindSequenceOnDiskPad

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* cpp - Fix FrameSet::inverted() memory leak
* cpp - Fix framesToFrameRange() bad formatting with 2 frames

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* go/cpp - Implement UDIM style syntax for padding
* go/cpp - Fix parsing error by relaxing acceptable characters in extension component (align with pyfileseq)

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* go - Prevent single file path from being appended more than one FindSequences*
* go - Fix FindSequencesInList formatting bad sequence string for single-file sequence with ambiguous range-ish pattern
* go/cpp - Refactor FindSequences* to use optional frame range pattern when detecting single frame paths

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* Split the FindSequencesOnDisk logic to expose new FindSequencesInList function

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* go/cpp - Add support for printf (%04d) and houdini ($F04) pad syntax formats
* cpp - Prevent segfault when calling getters on default constructed FileSequence; return default values
* cpp - Define FileSequence::operator bool() to return FileSequence::isValid()
* cmd/seqls - Update to a faster parallel filesystem walk library
* go - Update to Go 1.15.x