revise exported api

There are a lot of unnecessary exports in this package

# mxarray
export MxArray, mxClassID, mxComplexity,
       mxclassid, data_ptr,
       classid, nrows, ncols, nelems, elsize

export is_double, is_single,
       is_int8, is_uint8, is_int16, is_uint16,
       is_int32, is_uint32, is_int64, is_uint64,
       is_numeric, is_complex, is_sparse, is_empty,
       is_logical, is_char, is_struct, is_cell

export mxarray, mxsparse, delete,
       mxcellarray, get_cell, set_cell,
       mxstruct, mxstructarray, mxnfields, get_fieldname, get_field, set_field,
       jvalue, jarray, jscalar, jvector, jmatrix, jsparse, jstring, jdict

# engine & matfile
export MSession, MatFile,
       get_default_msession, restart_default_msession, close_default_msession,
       eval_string, get_mvariable, get_variable, put_variable, put_variables,
       variable_names, read_matfile, write_matfile,
       mxcall,
       @mput, @mget, @mat_str

We should definitely remove
mxClassID, mxComplexity, mxclassid, data_ptr, classid

nelems can be merged into Base.length
elsize can be merged into Base.sizeof

matlab_eval rename to meval
mat_str rename to m_str
mexcall rename to mcall

Start by locating the package's export declarations and searching for references to the symbols and rename candidates listed in the issue. Check the package test suite and public API usage before removing exports or changing names. Done means the intended exports and shorter names are implemented consistently without breaking existing package checks.