[indic] Handle "Cantillation marks for the Samaveda"
[indic] Support more extended Devanagari tone marks

Also adjust U+0953,0954 handling.
[indic] Allow up to three tone marks

According to Roozbeh, there are valid combinations in Unicode
proposals for up to three.  Previously we were allowing up to two.
[indic] Recategorize U+A8E0..A8F1 as OT_VD

Up to two of them come after all OT_A characters.
[indic] Treat U+1CE1 as a tone-mark too

It's spacing, but otherwise the same as the other ones.
[indic] Support U+1CF5,1CF6
Minor
[indic] Support U+1CE2..U+1CE8
[indic] Support U+1CF2,U+1CF3
[indic] Rename avagraha cluster to symbol cluster

In anticipation of adding more characters to that class of clusters.
[indic] Always start new syllable for Avagraha

In fact, the previous grammar was ambigious.  No functional
change.
[indic] Simplify grammar

No functional change.
[indic] Support U+A8F2..A8F7,1CE9..1CEC,1CEE..1CF1
[indic] Support U+1CED
[indic] Treat U+0A72..0A73 like regular consonants

Unicode 6.x IndicSyllableCategory categorizes them as
placeholders, but they can subjoin.
[indic/myanmar] Refactor a few macros
[myanmar] Use OT_NBSP instead of OT_DOTTEDCIRCLE for OT_GB

No functional change.
[indic/myanmar] Rename OT_NBSP to OT_PLACEHOLDER
[indic] Treat U+00D7 MULTIPLICATION SIGN as placeholder
[indic] Treat U+002D and U+2010..2014 as placeholders