fix warnings

remove unused "params" related code from regex

some structs and functions had reference to the params
feature of tre that is not used by the code anymore

fix regex on arm

TRE has a broken assumption that wchar_t is signed, which is a sane
expectation, but not required by the standard, and false on ARM's ABI.

i leave tre_char_t as wchar_t for now, since a pointer to it is
directly passed to functions that need pointer to wchar_t. it does not
seem to break anything. and since the maximum unicode scalar value is
0x10ffff, just use that explicitly rather than using the max value of
any particular C type.

upgrade to latest upstream TRE regex code (0.8.0)

the main practical results of this change are
1. the regex code is no longer subject to LGPL; it's now 2-clause BSD
2. most (all?) popular nonstandard regex extensions are supported

I hesitate to call this a "sync" since both the old and new code are
heavily modified. in one sense, the old code was "more severely"
modified, in that it was actively hostile to non-strictly-conforming
expressions. on the other hand, the new code has eliminated the
useless translation of the entire regex string to wchar_t prior to
compiling, and now only converts multibyte character literals as
needed.

in the future i may use this modified TRE as a basis for writing the
long-planned new regex engine that will avoid multibyte-to-wide
character conversion entirely by compiling multibyte bracket
expressions specific to UTF-8.

initial check-in, version 0.5.0